I agree with the sentiment that neither are meta and both are skippable. But I like comparing characters so will do my breakdown anyway.
Killmonger is probably better head to head. But Deadpool is more unique. And in a tier that’s pushing 50 characters I will usually choose the latter. If most of what you bring to the table is done by others, I get less excited about chasing (I say “less excited” because I chase everyone regardless).
Killmonger’s most unique characteristic is his black passive. It is awesome when the enemy hits one of those soul crushing cascades and you reclaim a bit of your footing when they are punished for matching crits. But the power often does a whole lot of nothing and (as has been discussed in other threads), the whole locking/unlocking thing is not as good as originally thought- and can often be helpful to the enemy.
Aside from the unreliable passive, he’s a solid two-nuke character who is somewhat boring- similar to Havok. Though the best comparison is 5* Parker. Both have hard hitting reds that spam crits, and both have a second nuke. Spidey’s is fueled by webs (which he makes). Killy’s is fueled by traps (which he makes).
Deadpool isn’t the straightforward beat stick Killy is, but is more of a support/defensive hybrid character.
Deadpool’s red is similar to Killmonger in that it’s great, but the tier is already stacked with awesome red abilities. And neither of the “extra stuff” are exactly unique to the tier. Ghost Rider stuns on red and (as mentioned) Spidey spams crits on red. Though Pool’s red and entire kit are a much bigger upgrade over Ghost Rider (read: he’s not a liability) than Killmonger is over 5* Parker. So there’s that.
Deadpool’s blue is a way better passive than Killmonger’s black for sure. Free AP, AP denial, and boostable passive damage for matching the right color? Pretty awesome. It also will trigger a lot more and be either something to chase or something that’s “nice if it happens”.
Finally, black is maybe my favorite power off the strength of how cheap it is. I know that detractors say you have to take damage to deal damage and that isn’t ideal, but you can pair him with other characters that can absorb hits and heal to mitigate that. Daredevil, Heimdall, Loki, and Phoenix for example. And even if you don’t pair him with a true healer. His cheap black means until he dies, he’s never out of the fight and can pretty much one shot someone if he’s on the ropes himself. That’s a dangerous power to be floating around out there when trying to finish a fight.
Basically because Deadpool’s passive and black ability are like nothing we’ve seen in the 5* tier, I’d vote Deadpool.
Plus… Spicy tokens! Sure you can get them with a single cover Deadpool in Deadpool’s Daily or trivial PVE nodes, but, having the option to win PVP matches with a champed Pool just gives more options there as well. Nice bonus when looking at the two characters and deciding who is more worth it.