Both versions of the game run at 720p, with both versions also offering the option to upscale this to 1080p. Neither version is "higher resolution" than the other.
The extra content details aren't yet confirmed except one - it's exclusive to Xbox 360 and PlayStation 3 will not be getting any whatsoever. I'd imagine it will be something like an extra bunch of missions, some cars and suchlike. It's not going to be like a sequel, just really an extension.