If you go with Kindle you should have no problem converting epub to Mobi through Calibre. I have a PRS-900 and my husband has a Kindle 2. We convert a lot of books and so far no problem.
Personally, I would not give up the touchscreen and general elegance of the Sony for the keyboard of the Kindle, although there may be good reasons. I have seen the new Pearl e-ink and would love to have it (over my 900). I would also love to have the translation dictionaries that I understand come with the 950. Speaking of dictionaries, have you tried navigating to a word to look it up on a Kindle? Maybe the K3 has made this easier, somehow, but I love the double-tap ease of dictionary use on the Sony.
Is it really true that the 900 cases don't fit the 950??
Best of luck to you -- any of your choices are winners, IMHO!
