I'd take Ultron easily. The sigs will come little by little, but by rarity he's the obvious choice. I'd never take someone who's on the basic because it may become your next pull and believe me, that feeling you get by pulling the champ you just used an AG on it's horrible and the 7* AG are super rare at the moment
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Dr. Zola
Ultron needs alot of sigs before it is really worth it, so you should only do that if you REALLY like him and have a bunch of sigs to invest in him.