There's much more to the comparison than chromatic aberration.
Yes, I know that. Here is a page of photos taken with the 250:
https://www.flickr.com/photos/yoyo31/sets/72157625042549743/My point is that I can see more chromatic aberration with the 350 than with the 250. The overall image quality of the 250 is higher, but that does not mean the 350 is a bad lens. It is a very good lens. The 280 APO is a superb lens. I am not sure whether there is a 1.4 APO teleconverter for the SL2 mount. Maybe you know. There were separate R and SL teleconverters, if I recall correctly. I have never used teleconverters.
The 250 is an excellent lens, the 280 APO is a superb lens, and the 350 is a very good lens. But they are all of different focal lengths and prices: you are not comparing apples to apples.