Either the Kindle Touch or the Nook Touch are decent purchases. It used to be that I would recommend the Nook over the Kindle, since the Nook could check out library books, but now the Kindle can too. Either one can side-load books, and it's trivial to convert epub to mobi or vice-versa, so that's not an issue. I'd say whichever one is cheaper or more aesthetically pleasing to you is the one you should get.
Take my advice and stay away from the Kobo Touch though. I had one and eventually got so fed up I sold it and bought a Nook Touch. It's so glitchy, and the text formatting (while better than its predecessor the Kobo Wifi) is still nowhere near as good as the Kindle or Nook.