I prefer the split dot screen, however I've found the fastest SLR to focus is a Zenit E. It has a plain ground glass focus screen, and reminds me of an old style TV picture tube. When you see the image clearly, you're set, and Zenit has made their screen more workable than most. As for declining vision and these screens, I think part of the problem is in loss of close vision. When we were younger, we could focus objects from close to infinity more easily. Now that we're not so young, we can't do that. The plain ground glass screen requires that your eye sees close images well, whereas the split dot works differently. Here is a bonus: for those of you who use "monovision" (and the list includes millions of people), use your weak eye to focus and your dominant eye to frame. This trick also works fine with rangefinders (whose bright spot also requires good "close focus").
Jon