I am going to go for Lesnar on this one.
Couture is the better fighter, and had he been 40, i could have seen him pulling it off.
He beats him on experience, skills, intelligence, fighting instinct, but i think Brocks size and strenght will wear him out.
Brock's been training and fighting for a while now with topnotch fighters/trainers, i don't think he's near his top, but i can see him winning.
Couture is 45 now, father time will catch up on him. I think this is the last big paycheck he wanted (even though its not against Fedor) and then call it quits.
This is how i see it: during the first round Brock is pushing the pace, Randy is able to hold him off. Nothing big happens.
Second round: Randy gets more and more tired. Brock ends the fight by ground and pound halfway through.