Cell Bio, no doubt. There truly is a lot of genetics on the MCAT, but not anything that requires an advanced course - most of the hard stuff comes down to recognizing the difference between recessive and dominant, autosomal or sex linked, etc. This is stuff you learned in Bio 1 and could easily refresh with a prep book.
Cell Bio, on the other hand, helps you to understand the Bio passages on a deeper level. Cell Bio covers everything from mitochondria to the nucleus to the cytoskeleton to exocytosis to... Cell bio covers many topics that will be on the MCAT, that although advanced info isnt necessary, it certainly helps when reading passages.