That is a very good question! Deadpool is…Deadpool. He’s fun to use and very wacky, but he’s certainly not the best character strategy-wise in Marvel: Avengers Alliance. On the other hand, Cable is actually pretty amazing in PVP if used correctly and excellent in everyday use. I’m about to level Cable up to 11 (DP’s currently level 10 as well) and he’s pretty great.
My advice: go with Cable!