The nook is tied to the Barnes and Noble store if you buy with wireless on the ereader but you can get files from numerous sites. The Sony is tied to the Reader store in the same way but again, you can get files from a lot of places. The Sonys are touch screen and have a reputation for being a little more sturdy, but they are more expensive and the smaller models don't have wireless. The Daily Reader does. These all use eInk (which looks like the pages of a book). The nook has the touch screen strip at the bottom and an eInk screen. The nook Color is really a smaller iPad that runs on Android and is slightly hobbled but a lot cheaper. It has an LCD and a shorter battery life (hours instead of days).
I, myself, have been going back and forth between the nook and the Sony readers for a while now.