The challenge for Penn won't just be his cardio strength, though. It's more a question of whether he has the heart. When you look at his matches against Frankie Edgar and St. Pierre, one thing is clear: Penn was mentally beaten. St. Pierre used his wrestling and extra weight to wear Penn out, and Edgar utilized his speed to frustrate the former champion. Against both opponents, Penn broke down mentally. His fans might argue otherwise, but you could see it in his face. Hopefully, he's closed that weakness in his game. A focused and strong-willed B.J. Penn can beat almost anyone in the welterweight division.Who will win the striking game?Both men have excellent chins and a solid stand-up game. However, I tend to favor Diaz here. He's bigger and stronger, and he has a longer reach. Diaz will likely be the busier fighter, and he'll definitely work the jab throughout the match to keep Penn at a distance. However, Penn's a well-disciplined boxer with a solid chin and power in both fists. He'll need to get inside on Diaz if he hopes to win the striking battle.I'm giving Diaz a slight edge here, but it really could go either way if the fight remains standing.Who wins on the ground?Yes, I understand that Diaz has a black belt in Brazilian jiu-jitsu under Cesar Gracie. I've seen him submit some great fighters, but there's a reason why Penn is known as "The Prodigy:" he's one of the best jiu-jitsu practitioners in the world. If this fight gets to the ground, I have no doubt that Diaz will be able to keep up with Penn, but I don't think he'll be able to submit him. I think Penn can beat Diaz on the ground, which is why I'm expecting Diaz to try to keep this fight standing.The winner is…I think Diaz wins this fight via decision. I don't think Penn will take this fight to the ground as often as he'd need to, and that's going to be the difference. Diaz's height and reach will be deciding factor in this fight.Derek Ciapala has been following MMA since the days when Ken Shamrock and Royce Gracie fought in the octagon. His favorite MMA memory is when he attended UFC 68 and watched Randy Couture come out of retirement to beat Tim Sylvia for the UFC heavyweight championship.Note: This article was written by a Yahoo! contributor. Sign up here to start publishing your own sports content. Updated Oct 28, 1:12 pm EDT