I'm sure you'd be happy with either a Gen3 or a Sony - both are excellent machines. The Sony is somewhat cheaper (especially with the low value of the US$), but the Gen3 does (IMHO, at least) somewhat more, in terms of its support for MobiPocket, dictionary lookup, etc. You really can't go wrong with either, though.
If you're into SF and Fantasy, check out
Baen. They are the "good guys" of the eBook industry, selling all their books in open DRM-free formats at very reasonable prices.