I sense that over time e-readers will fall into two broad categories: single- or mult-purpose devices. The Kindle and Nook appear to be a single-purpose device, simply trying to be the best e-reader devices for books, magazines and newspapers.
The Sony Reader and the Apple device (as rumored) appear to be multi-purpose devices--e-readers, music players and possibly more. With multi-purpose devices you may not get the best e-reader but instead get a pretty good e-reader combined with a pretty good music player.
Sorry Mike, last I checked every device you mentioned plays music. Well, except the Apple device. I assume it will play music if it exists - but as an unannounced product, it's hard to be sure. If Apple does release a tablet, it's no more a "Kindle Killer" than any other Tablet PC. Besides, if it runs a modified version of the App Store and it's easy to modify apps for it, presumably you can download the Kindle reader on your tablet, as well as the Barnes and Noble version and hundreds of other apps related to reading... I'm just saying!
Anyway, look for a better guide... maybe one that offers some information on how it's like to read on the device, or how much books cost for the platform on average... or, I don't know: how much the actual readers cost?!