Let's see, the 650 has a SD card slot and can play MP3s. Besides the differences you've already mentioned, there's just those two I can think of.
I chose to get the 350 over the 650 (though the latter had red -- I LOVE red!), but would the 5" size of the 350 be okay for you? You mentioned earlier eye problems, and although I read at either Small or Medium, my own opinion is that reading on a Large or XL on the 350 would mean the irritation of constant page turning. That really is the one reason why I might push the 650's 6" over the 350, so try your hardest to compare both models in person. I think there's several stores, not just Sony Styles, that have demo units across the USA.
Also, apparently it's a heavy shopping season in America soon? I don't think the new Sonys would be discounted by much, but there may be some competitive bundles around.
This 350 + cover is an example.
Anyway, I hope whatever you end up with makes you happy.