If you wait for the
EZ Reader Plus, which is the upgraded 6" EZ Reader Basic, there will be little to choose between the two except screen size. See the
E-book Reader Matrix for more details. The Pocket Pro is the best value for the money in 5" Readers, and the Plus is the best 6" under $250. The PocketBook 360 is in your price range and worth considering. The Kindle 2 and the Nook are nice devices, for a bit more money than you want to spend (particularly since both need covers, which are standard on the EZ Readers).
Note that Calibre can convert practically any ebook format to any other format, so the native formats are not necessarily a good deciding factor. Having many formats supported can be convenient, but on the EZ Readers in particular not all formats are supported equally well.