I use both types and both have their advantages and disadvantages.
Let's compare two leneses, a Nikkor ED 180mm f/2.8 and a 60mm CCD Guide scope, that I am guessing is about 220mm in focal length (maybe a bit more) and I am guessing around f/4.1 (about 250mm).
The Nikkor is an ED scope does not have to be used with long pass filters and maintains much of its sharpness, while the guider has a cemented doublet, and benefits from long pass filters. Under light polluted skies there is no advantage here for the ED Nikkor, but under dark skies, the Guider needs to be filtered, while the Nikkor does not have to be.
The Nikkor is very sharp off axis. It is a lens that was designed to provide excellent illumination and corner of frame performance over a 35mm full frame. The cemented and unflattened Guider is slighly comatic (if it were air spaced, this could be corrected) and has some field curvature, but it is not terrible, just not as sharp as the Nikkor.
The Nikkor, being f/2.8, is a bit brighter on nebula. Not a lot, but enough that you can see it is a faster system.
Now for the advantages of the Guide scope. First it is lighter. The Nikkor weigh 32oz vs 25 oz or the guider.
The ergonomics of the guider are superior. SLR lenses have a focuser ring and it is easy to move that ring and while hand holding, on this particular Nikkor lens, the focus is done by a large rubber knurled ring, and it is easy to disturb that ring while panning around. The guider by comparison has a microfocusing ring at the rear so that the entire tube of the guider can be used as a hand hold or a place to mount rings and nothing disturbs the focuser because it is very near the front of the NVD. Also, the micro-focuser is better for getting exact focus.
The Guider also provides a bit more magnification than you could probably get in a hand held SLR lens that would be light enough to hold. The 50mm is much lighter and closer to the Nikkor in focal length.
The guider, when used at prime focus, puts the filter on the front of the 1.25" nose and it is quicker and easier to change the filter and depending on the guider, it might even be possible to mount a filter wheel or filter drawer, though I have not done this to mine.
The guider costs a lot less than a good SLR lens.
Now the important thing is that it is not an "Either/Or" proposition. I have two SLR lenses that I use a lot and I have the guider that I use some. One of my most used SLR lenses is a Vivitar SMS f/4.5 70mm to 210mm zoom that cost me $30, including shipping. It is lighter than either of these lenses (19 oz) and while not great for nebula, it is fine for brighter nebula but being a zoom, it is a really nice general use scope for working the dark rift and being able to locate features then zoom in on them. The Vivitar Series 1 is f/3.5 and is a very respected lens, but it is considerably heavier than the Nikkor 180. Still, you get a zoom and more speed, but the Nikkor is already pobably as much as I want to hand hold.
The SLR lenses though are typically better optically and faster, but heavier, more expensive, and a bit more complicated to use due to filter changing.
I use them all and find that all have their strengths.