I would recommend going to
www.inkmesh.com to see what titles are available where. Inkmesh searches across bookstores, so you can see both price and location. The price part doesn't matter as much these days with agency pricing.
If you can wait, there are some new readers coming out in the next few months. The Sony T-1 was just announced. It comes with WiFi and a price of $149, much lower price than previous versions. There are also rumors of a Kindle 4 coming. I suspect that the Kindle seems to have a larger screen because the keyboard makes the whole reader seem larger. All of the 6" readers have 6" screens. The Pearl screens, such as the Kindle and Sony have, really are a lot better than the older epaper screens.
If you're opposed to readers that try to lock you into a book store, you might not like the Nook. Its software sends non-B&N books off to their own smaller partition and doesn't manage them as well as the B&N books. Rooting a Nook removes those limitations, but it sounds like you don't want the hassle of doing that sort of thing. Personally, I really like the Sony software and how it manages my books. Kobos and PocketBooks both have lots of fans here.