About 4 months ago I would have totally said Halo, but Hunters blew my mind.
Pro's and Con's
Halo
--Pros
●Great Multiplayer as well as single player
● Over rated as hell, so you know theres SOMEONE to play with.

● A great arsenal of weapons to play with.
--Cons
○ Shits to big, and too slow, pretty damn annoying. >_>
○ Not much better than any other shooter, just way fuckign over-rated.
○ Only 2 of them..
Metroid
-- Pros
● Best graphics for the system.. On the gamecube version, and the DS counterpart.
● Nice and fast.
● More creative, with the alt forms and affinity weapons; They add a lot more to think about.
● More levels than halo

● 7 diffrent hunters to choose from.
--Cons
○ Only Hunters was any good. I must admit, Prime 2 was pretty gay.
○ A limited arsenal... A total of 13 difrent weapons, iirc.
○ Unless you have WiFi, you ain't actually playing no one.

I think it's obvious where my vote lays, though I love them both. Before you just go lolz halo ftw! just b/c it's halo.. Check out just what the DS can actually do

http://www.youtube.com/watch?v=H7waHTMfkt4
http://www.youtube.com/watch?v=NAJXgQGjE78 xD Numa Numa
http://www.youtube.com/watch?v=2-M4FThRmC4
The last one is an impressive trailer