The biggest difference, I think, is in the book formats. The Sony Readers support EPUB, which currently seems to be the standard. Amazon mainly uses its own proprietary format (and doesn't support EPUB), though there are several places that offer free books formatted for it (http://en.wikipedia.org/wiki/Amazon_Kindle#Content_sources).
As for text-to-speech, Kindle offers it, but only on certain titles. It's pretty clunky - here's an example:
I can't honestly imagine listening to that for the length of a book - give me a mediocre audio book over that any day!
The biggest difference, imho, is that the Kindle 2 has a lot more features than what you're looking for. Here's an article with the comparison points: http://www.wired.com/reviews/product/giftguide_ebook
On another note, there's rumors of an Apple ereader coming out in spring. But, of course, being Apple, it will probably be exactly what you need but hideously expensive :glare:
Feel free to pm me if you have more questions about it - it's something I was seriously researching before deciding to stick with my Sony prs505 reader.