I would say BO2.
Less hackers, more "fun" to play.
More maps (soon) I suppose.
And better weapons.
I got bored of MW3 very early.
BO2 I'm still playing. (Maybe because I play with someone )
So yeah, my suggestion is BO2. MW3 is a bit out-dated
Since you're on a hacking forum.
MW3, because it's easier to hack there