B.J. Penn has headlined nine UFC events since September 2006. | Photo: Dave Mandel

A topsy-turvy series of events led to an unlikely main event at UFC 137 on Saturday at the Mandalay Bay Events Center in Las Vegas.In one of the more intriguing showdowns of the year, former two-division champion B.J. Penn takes on Nick Diaz , who returns to the Octagon for the first time in almost five years. Diaz will enter the bout on a career-best 10-fight winning streak and will try to get back into the good graces of UFC President Dana White with a memorable performance.Sherdog.com recently touched base with a number of professional trainers and fighters to gauge their opinions on the UFC 137 “Penn vs. Diaz” main event:I think Diaz wins. B.J. has more experience with tough fighters, but Diaz has more heart. I think Diaz strikes better than B.J., and the ground is almost on the same level. Diaz wins by TKO.I think B.J. wins.These two fighters are both so closely skilled. I don’t know who to pick.B.J. is a lot better than Diaz in almost every department. I don’t think Diaz has faced anyone as talented as B.J. He is a legend and a legend for a reason. I think Penn can stop Diaz on the feet or on the ground. The only chance Diaz has is if Penn gasses out for some reason, but I don’t see that happening. I see Penn taking this one by a finish in the second or third round.I think B.J. wins. Nick’s hands are good but not as fast as B.J.’s, and I think that B.J.’s punches have more power.Penn wins.Great fight. I like B.J.’s style, but I’m always wondering what mood he will enter the Octagon in, so I see Diaz as the favorite, despite being technically inferior in all aspects of the sport. He’s at a good time in his career. Motivation and good conditioning can make the difference for Diaz. He wins by unanimous decision.This should be a great fight. Standing, B.J. should be a more powerful striker, but Diaz will be throwing punches in bunches. Diaz will have a significant conditioning edge, and they should be very equal on the ground. B.J. does have slightly better wrestling. One strategy B.J. could follow is to take Diaz down and hold him down on the ground. I doubt that Diaz will be trying to get back to his feet because he is so comfortable off of his back. This could cost him rounds, as I don’t think he will be able to submit Penn from his back. The likely outcome of this fight will be that both fighters will stand and bang. I think Diaz will outlast Penn in the standup and should record a TKO late in the third round after a tough and bloody bout.Diaz’s pitter-patter punches and long reach will wear Penn down to get a unanimous decision win.I think B.J. will win by knockout or submission. This fight will be high-paced. Diaz plays too much when he exchanges, and Penn will exploit it. They are both otherworldly on the ground, but I still think Penn has the advantage. I can’t wait. It should be a war.Diaz is very tough, but I think Penn wins by knockout or submission. He has a more complete game than Diaz. Nick has good boxing and a good ground game, but B.J. is more complete than him.This fight is gonna be a war. I’m going with my boy, B.J., on this one. I think he has something to prove, and I can’t wait to watch.Nick wins.If B.J. fights a smart fight and takes Nick down, I would bet on B.J. But if he chooses to fight a standup fight, I think it would be like flipping a coin on who is going to win, with the edge going to Nick. Nick is busier with his hands and has the longer reach. Both guys have great chins. I do think B.J.’s Brazilian jiu-jitsu is better than Nick’s, along with his wrestling.This is a fight I’m very excited for. I’m going to take Diaz to win a decision.B.J. wins by decision.Height, weight, reach and power advantage go to Diaz. The outcome will be a unanimous decision for Mr. Diaz.Penn wins with a rear-naked choke in the second round.This is a horrible match up for Penn. Diaz is the best BJJ player and striker that Penn has faced in the UFC. Outside of that, Diaz is a very long southpaw with a shotgun of a jab. B.J. has been able to outstrike most guys in the UFC, minus Frankie Edgar , by using his long jab and quick combinations. Unfortunately, he will be on the receiving end of that this time. Diaz has an iron chin and is nearly impossible to hurt. Adding to that is [the fact that] Penn will not have much of an advantage, if any, on the ground. This will be a standup war, and, in my opinion, Penn will be stopped by strikes in the cage. Diaz will earn another title shot with an impressive TKO over a game but undersized Penn.That is for sure a main event fight. I’ll pick Diaz for this one, in a split decision.Damn, my prediction was that Georges St. Pierre would get injured, forcing Penn-Diaz to the main event.B.J. has shown his boxing skills have been much better than most and is always a favorite in the jiu-jitsu department. But in this case, B.J. is going up against an opponent that has better boxing skills than he has, and Diaz has enough jiu-jitsu skills to fend of his submission attempts. On top of this, he has to deal with a huge size disadvantage. Diaz wins by TKO due to strikes in the third round.Penn wins by knockout.Diaz wins. This should be a really good and interesting fight. I think Diaz’s nonstop attack will make it too difficult for B.J. to get off.This is an interesting fight. I see Diaz getting the best of Penn in the standup. And neither of these guys are going for the takedown, so I say Diaz by decision.I think this will be a very busy, as both are very technical with good ground games. It’s a fight where the impetus of youth meets the experience of age. I’m betting on a victory for B.J.Diaz-Penn was the real main event for a lot of the hardcore UFC fans anyway. When Diaz loses fights, it’s usually by being out-positioned and wrestled. I don’t think that Penn is going to use that game plan, and I don’t know that he can actually use wrestling to win this fight. I think this looks like a typical Diaz fight where he comes forward nonstop and uses his hands to pepper Penn. Penn has solid MMA boxing also and might land some shots with his straight right, but I think the Diaz pace will wear on him. B.J. isn’t known for his cardio, and that could play a factor. On the ground, most people would say Penn is better, but Diaz’s BJJ is very underrated. I say it’s even if the fight hits the floor. My pick is Diaz by decision, with Penn surviving but taking lots of shots courtesy of Diaz’s reach advantage.I kind of feel bad these two are trapped in this situation, but it will be a fun fight to watch. B.J. seems a little sharper in all categories, and I think he will win at least two of the three rounds. Penn wins this fight by decision, and they both win at the after party.Well, I think that Nick is really on a roll right now and I’m not sure where B.J.’s training is at, so proper preparation and tactics will play a big portion in the outcome. Both are so talented and fight worthy. B.J. has good standup and doesn’t get hit much in his fights. Nick loves to slug and is good on the ground also -- better if he is on top. So whoever gets the takedown will have the advantage. This will be a very interesting matchup.I’m betting on Penn.1313