I'm all about Apollo on this one.
This is a classic matchup of the boxer versus the brawler. As much as we can speculate that Clubber Lang studied Rocky's style prior to their initial fight, we all know that Clubber Lang is a brawler by nature. His style of that of pure power boxing, intimidation, and animalistic tactics in the ring. In both fights with Balboa, Clubber did most of the advancing in the ring, coming straight-forward at Rocky, leaving him susceptible for a skilled boxer to counter-attack against him. That skilled boxer is Apollo Creed.
Clubber's biggest downfall in this bout would be that he fights with too much emotion. Both of his fights with Rocky were anger-driven because it's plain to see that Clubber Lang has a short temper. Obviously, any fighter that is fueled on raw emotion makes mistakes in the ring, especially against an experienced fighter. Apollo Creed isn't someone you want to make stupid mistakes against. He can capitalize on those mistakes and break through the other fighter's defense like a hot knife through butter.
Also, considering the fact that Apollo Creed is very flashy and talkative in and out of the ring, he is a psychological fighter that can defeat an opponent before they even step into the ring. (Just look at how much he messed up Rocky's thought process in Rocky 1 and 2, prior to their fights.)
Another point about Clubber Lang that we need to take into consideration is the fact that Rocky did not train properly before his first fight with Clubber. If you recall, Rocky's training regimen was similar to Floyd Mayweather's in his recent fight against Marquez. He trained publicly, in front of cameras, newscasters, and the general public and didn't take the fight very seriously. Combine that with Rocky's stress about the fact that his manager and mentor was on his deathbed and it's plain to see that Clubber defeated a Rocky Balboa that was only about 60% prepared for the fight.
This fight is completely catered to Apollo Creed's experience. I'm sure that during his rise to the top he had to deal with many contenders that were similar to Clubber Lang. But Apollo doesn't make mistakes in the ring. He is a product of the system of boxing.
If Rocky Balboa dropped Clubber Lang in two rounds, Apollo can drop him in one.