First, I agree that a potential B+ or B or even a B- is nothing to withdraw over. Withdrawing from Biochem and then telling med schools you weren't doing well or were busy studying for the MCAT is probably going to look bad. What were you doing other than MCAT studying that you didn't have enough time to prepare? I would have made acing my classes first priority, MCAT studying second. You can always reschedule your MCAT date. I'd rethink your study habits in general so you bump those B's and B+'s up to A's. Just from what I've read, taking only 12 credits and saying you didn't have enough time to study is not a good sign.
If I were you, I'd stick with biochem and put my time and energy into my college grades and get them where I want them to be, and then worry about the MCAT. I'm not saying stop studying for the MCAT entirely until your grades go up, but make your coursework top priority, and figure out how you can improve your study skills.