St. Pierre all the way
I'm a fan of Hughes, and he is no doubt the greatest welterweight in MMA history, but St. Pierre will win this fight for the following reasons:
1) The Gracie factor: I have nothing but respect for Royce, but I won a hundred bucks by betting a couple of suckers who heard Gracie was undefeated in the octagon and figured he would submit Hughes. I told them that Hughes would destroy him in 1 round and they were outraged at this claim, but anybody who really knows MMA well, knows that Gracie was highly over matched against Hughes. And it this was the fight that made Hughes seem absolutely unbeatable in everyone's minds. Gracie may have been undefeated in the Octagon, but that was 12 years ago when almost all the fighters he beat (shamrock aside) wouldn't stand a chance against the likes of Penn, GSP or Hughes. Also, just because he was undefeated in the octagon doesn't mean he didn't get his ass kicked a couple of times, particularly by Sakuraba. He was past his prime and even Hughes stated that it wasn't one of his tougher fights to beat up on Gracie. This fight made Hughes seem like a God...but he is very beatable, as BJ Penn proved. Also, Gracie was a bad fight to have before GSP, because Hughes never even got hit and he barely broke a sweat. GSP will hit him very hard, many times. This will be a war, but just like Diego's heart and will took over in the last 2 or 3 minutes of round three against Karo, GSP's insatiable hunger for that title will give him the edge.
2) The history books don't tell the whole story: When you look at the record books, you see that Hughes beat St. Pierre in a first round submission (and you might think it was a quick easy victory for Hughes), but if you actually saw the fight, you'd recall that St. Pierre was winning until he messed up with a few seconds left in the round and got caught. He will not make the same mistake twice.
3) The Rocky III syndrome: Matt Hughes has the belt, has the money, has the legendary status, and he even has the victory over St. Pierre already. No matter how much of a competitor he is, he has nothing else to prove and it is literally impossible for him to be as hungry as St. Pierre is to win this fight. Remember how Mickey told Rocky: "you ain't been hungry since you won that title belt." Hughes is the consummate fighter/athlete/hard worker, but he has absolutely nothing to gain by winning this fight, other than to further the stories of the legend he already is. And because he has all the things that St. Pierre is hungry for, that is why St. Pierre will risk death to win this fight, and he will certainly risk a broken arm with 1 second left remaining in the round and he won't tap out, the same way Gracie wouldn't when his arm was hyper-extended by Hughes.
Bottom line: Hughes is the man and I still give him a very good chance to win, because anything can happen in a fight between two tremendous fighters like Hughes and GSP, but if you're a betting man like I am...go with Georges St. Pierre.
On a sidenote: who would like to see this card for UFC 65, 75, or whenever: Sylvia vs. Fedor Emelianenko
Liddell vs. Wanderlei
Franklin vs. Machida
Arlovski vs. Nogueira
Hughes vs. Kid Yamamoto