Gas tank aside, I think my logic was that I believe Cain's fighting style is to apply his own force onto the opponent and Nog may be in a size/strength category where Cain could do that, but he couldn't be as effective in applying his own force against someone as size/strength/powerful as Brock.
On the other hand, I think Big Nog's fighting style is to apply his own force and/or really exploit the opponents mistakes, if forcing himself on the opponent isn't the way to go. I don't see Brock being beating with force from either of these two fighters, but I can see Big Nog making good of an error, whereas, I don't see Cain as that smart and capable in that area of a fighter. Therefore, I give the advantage to Cain v Nog, but I give the advantage to Nog over Cain v Brock.