i'd just wait for black ops then see if it's all glitchy and shitty like mw2 first.
if all is good then take your pick
Here's how you determine whether you should play Halo or COD:
Do you usually suck at first person shooters or suck at video games? Then get COD.
Are you somewhat good at first person shooters or video games and want to play something that takes skill? Get Halo.
I don't know if Black Ops will take more skill than MW2, but all I know is that MW2 is the easiest game ever, and it's hard not to be decent.
This is incredibly true.