I owned a Meade 12.5 inch F/6 RG for a number of years. Honestly, for large Newtonians, GEMs are a pain in the rear.
- In getting good planetary views, seeing is the number one priority. It all starts with the seeing. One wants a scope of sufficient aperture than on a good night, it is not limited by it's aperture.
- In terms of planetary contrast and detail, aperture is more important than focal ratio. This is particularly true for scope on a tracking mount. Pick a focal length that is ergonomically acceptable, pick the largest aperture that is affordable.
- Important are high quality optics.
- Thermal management is critical.
- The mechanical structure must be stable and free from vibration.
- A small secondary is of some consideration but a little bigger than the minimum means alignment is easier, the illuminated circle is larger and any edge issues with the secondary are of less significance.
My best views of Jupiter and Saturn were with "Junior", my 25 inch F/5 Obsession. The seeing in the high desert where Junior lived was rarely more than average so such views were few and far between. Our home in San Diego is often blessed with very good seeing and at time excellent seeing, under an arc-second is relatively common. A scope that big is not practical. For my backyard, this is what I consider my best planetary scope.
- 13.1 inch F/5.5 Starsplitter with a Robert Royce mirror. It is a robust scope for a 13.1 inch, it's heavy but stable, the secondary is right at 20% and it has Feathertouch focuser. It has enough aperture for the really good nights while still being ergonomically comfortable.
- Tracking: I parted ways with 12.5 inch Meade RG when I acquired the Starsplitter. The Starsplitter was a package deal which included a Tom O, dual axis aluminum Equatorial platform. For visual observation, I think EQ platforms are superior to GEMs. A good one is rock solid and retains the superior ergonomics of the Dobsonian. The mount is rated for a 16-18 inch and yet weighs less than 30 pounds and can be carried in one hand. Truthfully though, I actually prefer manual tracking, I like the intimacy and the issues with nudging don't arise until well past 400x.,
My backup planetary scope is my 10 inch GSO Dob. I've had it for 15 years, it has good optics and it's a quicker setup. I had an Orion 120mm Eon ED/apo for a couple of years but I found the 10 inch Dob was enough better on the planets and double stars that the Eon just sat in it's case so I sold it. The 10 inch on the dual axis EQ platform.
I think that matching the scope to your local conditions is important. I am about 4 miles from the Pacific Ocean and generally south of the jet streams. The flow off the ocean can mean very good seeing and so in general, I have good seeing enough of the time that I do not need to fight it because there will soon be another night.