The variation in weapon combos in CoD4 means you won’t have the typical, formulaic approach to killing enemies you do in halo. That’s why halo gets boring in about an hour in my opinion (much less if you’re playing online with people you don’t know). Plus, you can find a perfect set of weapons for your style of fighting, and have multiple sets. The ranking system unlocks more weapons as you become better at the game, whereas in Halo the weapons are comparitively static.
For a better online combat experience, go with CoD4.