Both hold a cold part in my heart. As much as I love Captain Cold, he is more gimmick than the real deal. Since Fries has to be kept at a freezing temp constantly, he would be the "victor." (Pun intended)
I guess no one has read any of the latest Captain cold stuff. Captain cold lives by principals and is one tough son of a bitch. He doesn't play games with anyone other than the speedsters like flash. And though his outfit is quite lame, he actually rediculously smart... So Mr. Freeze (who is just a head now) I say you go down. Besides that, the cap only has to mention Mr. Freezes wife and he'll start reminiscing about his lost love and BAM captain cold smashed in his flippin face.