I know this question wasn't directed at me, but I'm still gonna answer it...coz I'm a pirate b!tch!!

If it is to play as a sole treqqie, like against Napoli, then there is no argument. Yoann is better, since it's just his natural position. Little defensive responsibility, complete freedom etc etc...
If it's to play in the mid three, then Seedorf, coz Yoann doesn't convince me defensively. But yes, Yoann could learn to play that role well, maybe even better. But I still think he'd be wasted playing there (so deep). It's not his natural place.
Playing in the X-Mas. Possibly. I can't decide. I think Yoann would be able to play on the right while Kaka plays in his preferred left position, and that would balance our left-heavy attack. But you have to consider that for the 2 players who can play behind our striker in the X-Mas tree, there's Kaka, R80, Seedorf and Yoann. So again it would be back to the same problem of playing time. Seedorf vs R80 vs Yoann.
And knowing our luck, if Yoann were brought in, miraculously all our injury problems in attack would disappear. Borriello would be fighting fit, as would Pippo and Pato. While all our treqqies are fine as well.

Either way, it's a selection headache. I'm glad I don't have to make it.