CoD4. Better gaming expierence and more "fair" online play. I don't have to worry about the easy-grasping auto aim of Halo making my expierence unfun.
And, even though the M-16 is a bit overpowered, most guns are relativly balanced with thier pros and cons. no super easy, no skill Sparten Laser, Brute shot, or Rockets.
My expierence on Halo goes kinda like this
Find decent gun,
find player,
first person to get a shot on the other wins,
if other player has Snipes, Rocket, Needles, any nub gun basically, throw sticky and hope he doesn't dodge it.
die,
spawn in the middle of the enemy,
try to look like enemy long enough to run away,
die by snipes while running.
come back in middle of enemy,
get meleed,
repeat.