The 505 is not MPEG4 capable, and MPEG4 format channel will be coming online this fall. Eventually all HD channels will be MPEG4.
Besides that, the 600 has a 14 day IPG with NEW/LIVE program tags, cast info, original air dates and search functions. The 505 has a basic 3 day guide with no search functions. The 600 is about a quarter of the 505 in terms of size. The 505 has a UHF remote and component connections while the 600 does not have these.
If you go with the 505 you will have to upgrade to the 600 series sooner or later (probably sooner).