Dayum. Excellent question. Assuming your bio skills are reasonable (i.e. you can draw a punnet square, understand why an X linked trait shows up more in men than women, and can do a hardy-weinberg calculation), I vote logic.
Genetics consistently buys you one or two questions on the MCAT, and it's the type of genetics you will pick up in a general biology class or MCAT review book. A formal genetics class, while easy and enjoyable and completely mind blowing, goes waaay above and beyond the scope of the MCAT.
Formal logic, on the other hand, is useful not so much for the theorem proofs but how it gets you to think carefully and exactly what statements and assertions mean in the English language, and what conclusions you may draw from them.
When I review student AAMC performance as a tutor, I find that students miss far more questions in the VR section and the BS section due to general logical errors than errors in any specific topic like genetics. I flew through these types of questions myself, but that's probably because I minored in logic and then spent seven years working as a computer programmer.
