It depends on Needs and possessions. IF you have a nice bunch of mystic Sig Stones Magik is a beast when you pump stones into her. If you don't I would probably with Cap IW as well since he is very versatile. He does a lot of damage and is a great champ. Gwenpool is a decent option as well since her awakening allows her to eat special 3's.
If I had to choose I'd probably go Magik. She's a boss-killing monster, she has one of the best power control features in the game, and she's a strong defender placed in AW which makes her a dual use champion.
But if you're choosing between CapIW and Magik, that suggests you're looking for a use for a generic awakening gem. If you honestly don't know which of the two will give you the strongest benefit, I would hold on to the awakening gem. They don't grow on trees, not even the 4* ones, and if you aren't 100% sure which champ will provide the best immediate benefit I would wait until you do. Whenever you're about to burn a rank up resource that is very hard to get for you personally, don't use until you're sure that no matter what happens the next day, you'll still be happy with your decision. And that usually means you have to be ranking up or awakening something that you will immediately put to use improving your game substantially.
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But if you're choosing between CapIW and Magik, that suggests you're looking for a use for a generic awakening gem. If you honestly don't know which of the two will give you the strongest benefit, I would hold on to the awakening gem. They don't grow on trees, not even the 4* ones, and if you aren't 100% sure which champ will provide the best immediate benefit I would wait until you do. Whenever you're about to burn a rank up resource that is very hard to get for you personally, don't use until you're sure that no matter what happens the next day, you'll still be happy with your decision. And that usually means you have to be ranking up or awakening something that you will immediately put to use improving your game substantially.