I like both fighters, but Thiago is probably my favorite UFC fighter.
He's got excellent takedown defense, and I think he has a good chance at neutralizing GSP's speed by his leg kicks. Plus, he's huge, and will probably have a good 10lbs weight advantage in the cage.
Thiago by stoppage.
I know, I'm going out on a limb, GSP is definitely one of the best right now, but I think Thiago is also an elite level fighter.