It all comes down to perspective and of course budget always rules. That said, slowing down and really thinking about what you want to get into matters a lot and will help steer your choices. While the Lunt 50mm is a decent instrument, it has compromises that for some are deal breakers. Unfortunately budget tends to steer people right into these instruments for this very reason and they're decent for visual use but they're much more difficult for practical imaging. That said, imaging can certainly be done, it just takes a lot more fuss to get it operational for that in a way that you may actually like with results that you are expecting. Expectation is also a big part of this. What draws you to want a solar scope? What do you like to see the most? Especially during the solar minimum right now with low activity? Knowing what's interesting to you also helps figure out what it is you may wish to steer towards.
This is merely my opinion/experience with this, so please take it with a grain of salt.
Visually, any of these instruments are wonderful frankly. Looking through a 40mm or 50mm aperture Lunt or PST is actually quite nice visually. You get a full disc FOV and can see the main structures. They're small and fairly light weight and rather easy to use with a little sun finder. Impressive to look through, especially as the maximum returns with more activity; during the minimum cycle (now) activity is low so there's less to see, but there's at least always something to see in Halpha, if even just a prominence here and there and it changes hours and daily rather significantly.
The issue is most of these entry instruments were not designed for imaging in mind, so while people find work arounds and get it to function, they do it with significant compromise. The Lunt 50's biggest issue is that it has a helical focuser and the imaging train is hard to use a camera with to be able to come to focus on the sensor, requiring a break down of the pieces of the blocking filter to gain some back-focus into the light cone and having to deal with the helical focuser for precise focusing and any potential sag/tilt which will induce issues too. The focuser is replaceable, but it's never inexpensive to change that to a quality focuser that can handle imaging loads with precision. You can double stack the 50mm later on nicely. The other compromise is that the focal length and small blocking filter means you have to use tiny pixel sensors to ideally sample the wavelength (656nm) but at the limit of the blocking filter's diameter, so its very limited in size (4mm to 6mm depending on which you get). That said, at least the sweet spot is a bit bigger than what the PST has, so its a bit more friendly to imaging full discs. To make the Lunt 50 a great imaging OTA, it would honestly require a few things over time: 1) new focuser, ideally a custom crayford, costly; 2) a larger blocking filter (I would shoot for 12mm; costly); 2) and of course a double stack module. From there, a good camera for it will not be inexpensive, because of the short focal-ratio, to properly sample the wavelength would take tiny pixels and most of these current tiny pixel cameras are not without issue (ie, IMX178).
The PST and it's double stack SM40II module is actually fantastic visually, a real double stack view (high contrast) for an entry price. Visually it is actually a nice instrument for this purpose. Imaging is a lot more difficult unfortunately. The focusing mechanism on the PST is an internal pentaprism that is moved by an internal screw with the focusing knob. That's it. A moving big chunk of prism. There's a lot of slop to it, shifting, and if it's not square there will be issues and these are notoriously not put together with precision in mind. The blocking filter is 5mm and cannot be changed stock. So the distance of the light path and the pentaprism focuser and then the eyepiece stalk to the blocking filter is significantly long, so imaging with the PST requires compromise just to get focus with a camera. You either are forced to use a barlow to bring the focus point away from the instrument, but you're still limited to that 5mm blocking filter size, or you have to use a specific camera with small pixels and a low profile adapter to allow it to have more back-focus travel to get in range to focus on the sensor. The transmission on the PST and double stack will be fairly low usually, again, designed to be a visual instrument and the sweet spot is very small, so its not ideal to image with, but can be done. Nothing can be swapped on this instrument, so you're stuck with its focusing mechanism and imaging train and blocking filter size. Its the most limited of all instruments in this category.
40mm aperture's resolution to 50mm aperture's resolution is a 25% jump. Going from 50mm to 60mm is a 20% jump. Going from 40mm to 60mm is a 50% jump in resolution.
Every incremental increase in aperture matters significantly!
Now, the 60mm instrument options are far more friendly to imaging. The Solarmax and Lunt options at 60mm, even the old ones, are much better than the 40mm/50mm options listed above. They have more after market options, blocking filter size options, focuser options, etc. They're frankly a much better base instrument to start with that can be added to over time as you please. For anyone seriously concerned about spending this kind of money and not being underwhelmed with their purchase, I would avoid the Lunt 50 and/or PST if your goal is imaging. Visually they can be satisfying. But for imaging, I highly suggest passing them up and moving towards the 60mm option scopes from Coronado/Solarmax and Lunt. These instrumenst are frankly just a lot better and more friendly to imaging due to being able to control and swap more parts of the OTA and imaging train for the purpose of imaging. So even a used old Lunt tilt tuned version, or an old Solarmax II series version would be better than a PST or Lunt 50 for this purpose. And the aperture really does matter a lot here, that 60mm is significantly more resolution than 40mm and 50mm.
Lastly you have the Quark option. It's at the similar price point. It is modular and allows you to pick the OTA which means you choose aperture, you choose focal-ratio, you choose the focuser. This gives a lot more freedom on which camera(s) are ideal for your setup, and of course, which overall scope you want to use. Visually they're good, especially for looking at large structures in higher resolution as this is the least expensive way to get larger apertures (and 70~102mm apertures in solar is actually very big). Yes, they require power and time to warm up to temp to be on band. About 5~7 minutes usually. Imaging wise, the Quark is a gateway choice to high resolution imaging if your seeing conditions support larger apertures. Imaging with an 80mm, 102mm or 120mm for example is a very different output than something only 60mm. So if your interest is single large structures highly magnified and higher resolution, this is an option that outpaces smaller apertures for this price point. Compromises will be that imaging a full disc with a Quark is very difficult, requires compromises to achieve due to the focal length (Quark's have an internal 4.3x telecentric amplifier, so you're always working with long effective focal length, which makes full disc imaging hard). This would require multiple OTAs to control FOV, such as a very tiny short OTA for full disc viewing/imaging and still requires focal reducers and spacers to accomplish that, with an effective 12mm aperture blocking filter, so larger sensors can be used (which is good for flexibility). And another OTA with larger aperture (and a really good focuser) to do high resolution or simply larger magnification views of the partial disc, with significantly longer focal-lengths. The other big compromise is that there's no commercial way to double-stack this without getting another HA filter of some kind and working it into the imaging train while maintaining aperture (the common way to double stack would be to have a dedicated HA telescope and use the Quark with it, effectively double stacking, so like the 60mm or 90mm options out there). There's also of course a few people who have used two Quarks together (one being the Combo version without the 4.3x telecentric internally) with passable results, but I don't suggest this at all really. The Quark really should be looked at as a single stack solution with higher resolution options due to being able to use larger apertures right away. This can be good for imaging. Visually it can be good, but since its single stack it will be lower contrast. There's always compromise in some way.
Personally, I vastly prefer visual of the sun with a double stack and smaller aperture, I like seeing the whole disc and all the features at once with high contrast. So for me, I much prefer to use my 60mm double stack visually than my larger scopes. I have larger scopes ranging from 80mm to 200mm in HA that are single stacks and I image high resolution often. But visually they're not as appealing with less contrast. So this is again greatly personal preference, someone else may prefer totally different options. But having a 200mm solar scope and all options below that, visually I frankly prefer to do visual with a smaller double stack. The contrast is the king for me. Imaging, I like full discs, but I also really like moderate aperture for resolution, so I like being in that 120mm aperture range for imaging and my seeing supports larger apertures fairly common here in Florida in the mornings.
So again, food for thought and take it with a grain of salt. It all comes down to what you want the most out of what you're looking to get into and what your expectations are.