Most expert opinion is moving in the way of Brock Lesnar now as the fight approaches. He's too big, too strong, too good of a wrestler, too athletic...and Randy is just too old.
There are so many key factors in this fight that it's impossible to address them all in one post, but I believe the deciding factor in this fight will be instincts. A great comparison is to look at how both followed up their knockdowns of Tim Sylvia and Heath Herring. Randy looked shocked at first, but immediately rushed in, landed several lefts, and took the back to try to get a choke. Brock Lesnar followed with...I don't know, a quarterback sack type maneuver. He ended up against the cage holding Herring in a headlock while he looked to his corner for advice and then figured out what to do.
The problem for Lesnar is his instincts aren't there. If Randy ends up on top in half guard, Lesnar is not going to be able to take 15 seconds, remember what to do in this situation, and then do it. He'll have to be ready to defend immediately, or he'll get pounded. I have no doubt Randy can recover and survive a bad shot, but for Brock, as soon as something goes wrong, I think it's over for him.
When Brock Lesnar trained for Frank Mir, he trained the escape to that leg submission thousands of times with great submission coaches. But in the heat of the moment, when Frank got it, Brock didn't even go for the most rudimentary of escapes. He felt the pain in his leg and just tapped. He panicked, and that is the kind of thing Randy Couture does not do.
If Randy Couture floors Brock and grabs his back, I have no doubt he'll easily grab a choke and finish, just because I don't think Brock will show the poise Tim showed that night. He will one day, but it's just too early. For Brock to win, he needs to finish early, or go a full 5 rounds without making a mistake. The latter is highly unlikely, so he needs to finish early.
I'm obviously not ruling out a Brock victory. If it turns into a pure wrestling match, it's Brock's fight. He could bust Randy's eye socket from the top and it will be over, he could catch Randy with a perfect punch...but the odds of Randy catching Brock with that punch are much higher.
In the end, Randy Couture is fighting a phenom, but the phenom is 2-1. He has never been punched in the face in a fight. He looked clueless in the clinch just 3 months ago. I believe Randy Couture will finish Brock Lesnar, and finish him early.