GSP should be favored, Penn the better gambling bet
Just by reading the thread I can tell about 90% of you have just recently started following MMA.
To anyone who is saying GSP kicked BJ's ass in the 1st fight, what are you smoking? GSP was a bloody mess, Penn was unmarked. If someone just showed you the photos of the fighters after the fight you wouldn't believe who got the decision.
GSP does have a significant size and strength advantage, he's a very big welterweight while BJ is a big lightweight but not a natural welterweight. But size and strength have never affected BJ (who has fought guys as big as Machida at 205 without it being an issue). He was able to beat Matt Hughes at the top of his game and he's much stronger physically now than he was back then. The strength difference will help GSP avoid takedowns and score some takedowns which will be key in judging if this fight goes the distance.
GSP has terrific cardio, the guy is a workout fiend. Penn cardio is vastly improved, but he'll need to come out strong early while he still has alot in the gas tank as GSP will outwork him in the final latter rounds.
GSP usually has great takedowns and GNP, but the problem is Penn is hard to take down, even tougher to keep down (he has insane flexibility) and very dangerous submission game. He almost got a omaplate and a gogoplata in the first fight and never panicks.
GSP has better kicks in the standup while BJ throws the quicker, crisper punches. The last few fights GSP has had the luxury of fighting mainly wrestlers with mediocre striking games (Fitch, Hughes, Koscheck) because of his suspect chin. BJ has heavy hands so I think GSP will go for alot of takedowns especially if he does get rocked, these takedowns will win him points with the judges but won't necessarily inflict any real punishment.
If Penn wins, he'll win do it by the 3rd round, otherwise GSP will take a decision in a 5 round war. The winner of the fight might look more busted him than the loser.
I'd say GSP has a 60% chance of winning, I'm betting on BJ though because the +150 odds are better than GSP -189 right now. A loss for either of these guys doesn't tarnish their accomplishments, we're witnessing two of the top MMA fighters of all time at their peak. Should be a fight to remember.