I have owned a Mewlon 210 for seven years and purchased a used Mewlon 250 (non CRS old model) since a number of years now. I didn't have enough overlapping time between them to do a side by side test, but nevertheless, here are the some comparisons.
- Optics : First of all, Optical quality in both scopes is first class ! Razor sharp images with excellent contrast, no diffusion and an sort of APO feeling that is amazing ! Really excellent optics !!
- Weight : The 250 is considerably bigger and heavier than the 210, 12kg for the 250 versus 8kg for the 210 ! Much more in fact than one would expect by comparing the diameter numbers. The 210 was fine on my EQ5, but it is not possible to mount durably the 250 on it. I had therefore to purchase another mount (iOptron IEQ45)
- Cooling : With it's removable back and despite being bigger, the 250 cools down more efficiently than the 210, and this results with images being more often stable.
- Light grasp : The 250 has more light grasp than the 210, and this is clearly visible on DSOs. As an example, M13 is resolved in both scopes, but it is more clearly visible in the 250, and you have a WOAH effect on that kind of objects that you don't have with the 210. Light grasp is also an important factor for planetary work at high magnifications, for viewing and especially for imaging.
- Resolution : The resolution difference between the two scope is not huge at all, but this coupled with more light grasp make everything more easy with the 250. You won't necessarily see more things with the 250 but you see them a lot better ! Views of Jupiter for example are more comfortable, you can easily see festoons in the SEB and NEB, details in the gread red spot or in the northern and southern caps. I can observe at 300x on a regular basis with good image stability with the 250, and I had some very good seeing one day and was able to push the magnification up to 600x on Jupiter : everything was so sharp and clear with excellent colors ! I was even able to see some features on Ganymede, thing that I was never able to do with the 210. On the moon, you can see approximately the same amount of detail with both scopes, but again it is a lot easier in the 250, more clear, more stable. Hadley rile for example can be entirely seen with no problem at all in the 250. It is a less confortable view in the 210. For planetary imaging, light graps and resolution make the 250 a lot better than the 210.
- Field of view : Focal length of the 210 is 2412mm versus 3000mm in the 250. This has an effect on the bigger accessible Field of view. The bigger objects like M81 are of course not accessible to both scopes. But there are some that are better adapted to the FOV of the 210 than the FOV of the 250 : M42 and the pleiades for example.
- Central Obstruction : I measured the secondary CO on both scopes : 70mm on the 210 and 80mm on the 250. As a result, the CO is not really smaller in the 250 : 32% versus 33% in the 210. The internet datasheets of the 250 claiming 28% is false (don't know why ....).
- Focusing : The 210 focusing system is the same as the one found on classical SCT, a moving primary mirror with a decentered push/pull focusing screw, so it does show some shifting ! This was annoying for me (especially for planetary imaging), so I added an electrical moonlite focuser to my mewlon 210 and after that it was perfect. It is not the case of the 250 with it's electrically movable secondary, no shifting at all and a very precise focusing.
- Diffraction spikes : The 210 has a 4 vanes spider resulting in 4 diffraction spikes, and the non CRS 250 has a 3 vanes spider resulting in 6 diffraction spikes. But the 210 spikes are much more visible than the 250 spikes. On the 210 you can see them when observing Jupiter for example, with the 250, they are invisible on Jupiter. In this regard, the 250 is much better.
- Dovetail : The 210 has a Vixen Dovetail which does the job well because the tube is not heavy. The non CRS 250 has a proprietary Takahashi dovetail and you will need to have the takahashi female part (provided with the scope) to mount it or replace the tube dovetail by a Losmandy one, which is not necessarily simple. The Mewlon 250 Takahahsi system is excellent and very secure but doesn't allow to translate the scope to equilibrate it, so the scopes has a magnetic movable conterweight system under the front of the tube to address this issue.
As a conclusion, I personnaly prefer the 250 to the 210 despite the weight issue as everything is lot more easy to see than in the 210. The Mewlon 210 is an excellent scope and I was very happy with it for 7 years. But the 250 is a superlative scope, and if the price and the weight is not a problem for you, I'd go this way.