I don't really know if you can compare the two. Not because they're totally different games, but because their entire approache to game design is fundamentally different. GTA:SA is about giving the player a crap-load of stuff to do and just letting them do it. There's a nice story that can be followed with some sweet Voice Acting but the real features of the game are just how many different things you can do (parachuting, jetpacking, dildo bashing, etc.)
Halo is about giving a small number of abilities that are highly highly tuned and polished. Gameplay-wise Halo2 adds 2 new features, dual wielding and hijacking. Whereas GTA:SA offers dozens of new gameplay types and experiences over its predecessors.
In terms of pure numbers, GTA wins hands down. However, when you consider that one of th changes that Halo2 brings is Live that really changes the balance. Live has totally altered the way the game is played. Instead of having to get a bunch of your friends over to have a big LAN match, or just do 4 player split-screen, you can have 16-player CTF matches in the span of a few minutes. That is a fundamental shift in the dynamics of the package as a whole. In contrast, while GTA:SA offers a vast number of improvements and additions, none of them makes a significantly large impact to the way the game is played.
So taking that all into consideration I'd say Live play really puts Halo 2 above GTA in terms of new features. However, if GTA:SA had included an online mode, even if not well implemented, would have won the features contest hands-down.