you know, if you're really weak in genetics, it might not be a bad idea to take the course so that if, by chance, you get any genetics related passages, you'd be fine on the real mcat. (also, keep in mind that it is genetics that kills most students on the mcat...ie. "oh my goddd..that test had sooo much genetics, i think i bombed it!")
on the other hand, you could take a chance, not take genetics, and just hope that it doesn't show up. the liklihood of them giving you some hardcore genetics passage is pretty low...(although it is one of the recent changes that the test writers have noted). but nonetheless, i'm sure a good background in regular bio should be fine for the genetics required.
by going with the biochem route, it will definately make the entire bio section seem a bit more familiar...esp if you have anything related to proteins, lab techniques (ie westerns, eliza..etc), or even some of the respiration and metabolism (ie. glycolysis and krebs/oxidative phosphorylation)...
in the end, i'd say i would take biochem, just because it would give me a broad preparation...while genetics is worthwhile, there is not too much that could show up on the real day that a bio course wouldn't have taught you already...
hope that helps...
biochem is what i say.