Going to be a good fight.
Fitch will be overwhelmed striking, so he will auto go for the TD. This fight purely depends on if Fitch can get the TD early or not. If he can't get it early, he's going to get beat up standing until he can get it.
If Penn comes into the fight with very solid condition, he'll be able to stuff the TD for a good while. Penn has some of the best TDD in the world and it took GSP a round and a half to wear Penn out and get him down, and Penn wasn't in good shape at all. Penn comes in good condition = Penn can stuff the takedown long enough to win the first two rounds, then there's no way Fitch can finish him in the third.
Hell, Penn can rock him and put him away, he does have ridiculous power when he connects right for being such a small guy.
Anywho, point is, if Penn can stuff the TD he wins, if he can't he loses. That's what this fight will depend on.