I'd go with the approach that yields the most legendary tokens soonest. Champ Mags now, then save up for 13 days and champ Deadpool. Unless one or the other is going to make a drastic impact to your ability to achieve progression or placement rewards *today*, the order doesn't otherwise matter. You'll have ample opportunity to champion most of your 3* roster by the time the 4* covers they'll yield become a factor. Getting more legendary tokens now increases the likelihood that you'll have a usable 4* or 5* character.
Thanks for all the info guys. I like the analysis of both of these heroes. So clearly Magneto is the better hero and worth promoting.
I guess my other question would be, is Deadpool even worth leveling much past 120 and promoting if I have a fairly solid lineup of maxed out three stars? I guess I kind of feel bad tossing four covers for him but I also don't want to feel like I wasted 97k Iso on leveling him if I don't use him.