Well, Graviton could turn parts of his body into those gravitonium tendrils, and he swallowed up both people and aliens with it.
As to the OP, I would back Graviton for one big reason. Both characters have really good damage/power output, but one of them is kind of a glass cannon, and it isn't Graviton. So, I feel like he has a better shot at taking Jean's attacks than the other way around.