Nobody is gonna finish anyone in this fight. I see Randy winning this, and I see him employing the same technique he used against Gonzaga and Sylvia. He outworks the bigger fighter, clings on, uses his wrestling and super human strength to just hold on and chip away at the other guy.
Sure, Lesnar has phenomenal strength and wrestling, but this is MMA and I don't think he can quite put it all together the way Randy can. Plus, Brock doesn't have the same striking/top control/GnP as Ricco or Barnett, so I doubt he will have Couture in that kind of trouble.
Thus I see Randy winning a very hard fought and meticulous decision.