Even though Lesnar is favored in some online books, I think Randy will take it, easily. Although Lesnar essentially takes Randy's main strength away, Heath Herring's main strength was also wrestling, and although Lesnar out did him, he wasn't really that dominating. So on paper, and using a bit of common sense, Lesnar is the better (and by better I mean bigger and stronger) wrestler, but if his performance against Herring is any indication of how he fares against other wrestlers, don't be surprised is Randy can pull something out of thin air.
Randy has the clear advantage striking, both offensively and defensively. Remember Tim Sylvia? The poor bastard who shouldn't have had the belt in the first place and is the reason Randy's in this crap situation at all? Yeah, Randy easily out struck him and effectively evaded him when necessary (i.e. head movement) in their fight (not to mention a first PUNCH knockdown). As much as I despise him, you've got to give Sylvia the striking advantage over Lesnar whose only real strike was telegraphed into next county and was against the supremely washed up Heath Herring.
So theoretically this should be a breeze for Randy. But I see two x factors. First, Lesnar's youth and general unpredictability. We really don't know if he'll be the same fighter we saw against Herring or worse or a thousand times better. He's new enough to the sport where he could be malleable enough to soak up his trainers' advice, but then again maybe not--see bronco riding and compare trying to teach Lesnar striking to trying to teach Kimbo Slice ground skills.
Second is Randy's ring rust. I wouldn't see this as a problem except for two things: Randy's age and what he was doing during his 14 months off. Although it is phenomenal how he won the belt at his age, his opponents weren't really that good: Sylvia-perhaps the most overrated heavyweight in the history of the sport and Gonzaga-a one hit wonder if there ever was one (though I wish the latter weren't the case...). Brock is younger, stronger, and quite frankly has more to lose in the eyes of the UFC management than either Sylvia or Gonzaga. I know Randy's a phenom, but by my calculations he's now 45, a elderstatesman in MMA if there ever was one.
And during his time off he was entangled in the legal disputes that unfortunately was the subject of much discussion during many of those 14 months. I would think that this would be so emotionally draining, but then again Randy was shooting movies during this time, so maybe not.
Anyway, to finally wrap this up, I think that although there are many conditions outlined above where Randy could falter, given his fight history and his reputation as supreme competitor, I doubt all of these contingencies will all end up being held against him. Brock on the other hand is just a giant (quite literally) question mark. A giant question mark (who is also a giant marketing pawn) versus an established Champion with merely some "but, if's".
To answer the original post's question: I prefer Randy (by strikes early RD 2).
Kevin (aka longwinded noob)