Fitch, no question. Kos' standup still sucks ass, he's fast and he's got power but his technique blows. He doesn't use angles, doesn't use level change combos, doesn't control distance, doesn't mix it up, and just relies on his power. Fitch's standup on the other hand is very under rated, he'll mix things up, use angles & level changes and go body-head instead of just head hunting, and he's much better at controlling the distance. But since he doesn't KO people and likes to take guys down and break them instead most people think his striking sucks, but that simply isn't true. His striking technically is quite good.
Wrestling wise it's probably even, Kos has the big power double which can take down practically anyone while Fitch has takedowns from the clinch that are just as good. If either one of them wants to take you down, you're going down unless your name is GSP. Once it goes to the ground, Fitch by a mile, way better BJJ game and no one grinds people down and breaks them the way Fitch does.
Then there's the mental game, the most important part. Kos has way too many brainfarts and stupid game plans. Such as let's go stand and trade with Thiago Alves till he kicks the legs out, then start trying for takedowns. Or standing & trading with GSP instead of going hard for takedowns. Fitch knows what he has to do to win, and every time he goes out there and does it. No matter how much the fans boo and criticize him, he's going to go out there, take the guy down and grind him out for the win, guaranteed. He's not going to stand & trade, he's not going to get dumb, he's not going to over-commit trying for a KO, he's going to mix it up just enough to drag the poor sucker down and work him till the clock runs out. It ain't pretty but it works damn near every time.