I gotta go with devils here.
Imagine the scenario: A seemingly endless horde of AvP Aliens, versus all the DMC demons we can think of. I think the demons have the upper hand.
The Aliens are quite agile, can climb walls, have a second mouth inside the first, sharp claws, a powerful tail tipped with a spike, and insanely corrosive acidic blood.
The DMC demons... Need I say more? Phantom can launch fireballs, Cerberus can summon shards of ice from the sky, Beowulf can batter things with his brute srength, Agni and Rudra wield scimitars imbued with fire and wind, and there are countless more.
You know what I'm saying? The demons are much more dangerous, and have a massive array of abilities. Plus, the lesser demons, such as the Hells and the Sins, number in the thousands. And on top of that, you throw the obvious choices, Dante and Vergil, the ultimate sword-totting, demon-slaying badasses. Now you have an enormous army of deadly creatures with a huge variety of weapons and powers.
Aliens: Strength in numbers. They win by overwhelming their enemies, who are normally underequipped, outnumbered, basically mediocre humans.
Devils: Truly huge armies of bloodthirsty monsters with a versatile and deadly arsenal of abilities. Any lesser demon can hold its own when in a group, and the greater demons, like Phantom, can effortlessly lay waste to entire armies.
Aliens ::::: Devils