I don't think it matters that much. Buy one and after you have some experience with it, you'll know what you want in your next EBR.
Alternatively, go to YouTube and watch video comparisions of different EBRs to get a good idea of how they differ in actual use.
To get you started:
http://kindleworld.blogspot.com/2010...s-another.html
http://www.youtube.com/watch?v=jgr_dGNW1sA
http://www.youtube.com/watch?v=Q43M3...eature=related