I'd read up on the differences between the two systems before doing that, it's not an upgrade, just a change of system, and you lose a lot more than you gain IMO. You will see quite a few people on here asking if they can switch back (they can't).
Some of the missing features:
Undelete, skip back after FFWD, setting series links for programs not in the TV guide (so on in the next two weeks), series link manager, series link+, wishlists, quickplay, suggestions, buffering and switching between 6 channels, radio channels (and therefore radio recording), individual list of recordings on each individual box, control over ethernet and probably a few other things I've forgotten. The 360 also makes it more awkward to navigate to your recordings and uses a lot of pictures that take up space and generally make the menus more awkward to navigate.
All you really gain is the voice control and profiles (which don't do much).