I'm not sure, but didn't Amazon just come out with a new DRM that is virtually unbreakable? I thought I saw something like that from Alf? I don't know because I don't have any Kindle books.
I think the Kindle is pretty good for what you wan't (Web browser sucks though) . The Nook and Kobo touch are nice too but the Kobo touch doesn't have physical page turn buttons which can be annoying from what I've heard (The Nook is touch and buttons) .
I haven't tried out the Sony so I can't give an opinion on it, except that it is Sony.
Remember that if you go with a Kindle you can buy from almost any other company that has the option to download the Adobe E-pub version, which you can strip of DRM and convert to MOBI using
Calibre and plug-ins. Same for library books but that might be illegal in your country.
But if you go for the Kindle you might want to wait a few months, because the new one is rumored to be Color E-ink and touch. So if it can wait a while you might want to.