Despite the fact that usually Marvel's characters copy a previous DC archetype, Man Thing came out before DC's Swamp Thing. And it has a cooler title. "I'll give you 3 bucks for that old giant-sized Man-Thing". Oh the laughs.
I say Man-Thing. Although, I don't know much about Swamp Thing. He's probably as fast as the flash, stronger than super man, has baked goods vision, unbreakable prunning shears and more, since he's DC.
They're fairly different characters despite apparent similarities. Man-Thing is nonsapient and those who feel fear burn at it's touch.
Swamp Thing is sapient and, when he fully realizes his elemental nature, quite powerful. Being *the* plant elemental of all of Earth, I think Holland is on a different level, and could likely 'tell' Man-Thing to surrender.