Part 3 is mostly sidequests, so about five/six hours more or less? The last dungeon is pretty big but you can skip half of it if you're not going for the Fell Weapons.
And about which version to get, the technical problems aren't really much of an issue, they're hardly noticeable if at all. That being said, and even though there is new juicy content, I personally think it's better to have it now and much cheaper. The game is already amazing and complete in 360, but if you can wait you'll get some more stuff to do.