I dunno if this is a fair comparison. If they were collabing with the same person, it'd be fair. But Lady Gaga's song was during her peak, and come on. It's Lady Gaga. She was gonna smash regardless.
will.i.am, on the other hand.. he needed assistance. Definitely needed a huge name on there.
I'm gonna have to go with Britney, but the comparison isn't fair at all. Gaga wasn't dependent on a huge music act as will.i.am clearly is; he wasn't going anywhere if he recruited just any random person, or going solo.