Adesua, you must be an amazing candidate to get into the MSTPs at both of these schools! I bow down to your brilliance.
I would pick Wash U if I were you (and it kills me to say that, as a former Columbian). While Columbia probably is better known among laymen, I have heard nothing but the highest praise for Wash U's research and especially for their MSTP. From what I have heard, Wash U and UT Southwestern are the two best places to go to do an MD/PhD, because the classes are so large and they have so much money to spend on you. You're a relatively large portion of the class, and I believe these two schools primary focus is on the MD/PhD students. Everyone I've ever met who's worked or studied at Wash U has gone on and on about how they really go out of their way to make sure you are happy and get everything you need to do whatever research you are interested in. And there is a big disparity between the two schools' NIH funding.
They may have resolved this already, but back when I was talking to my bioethics professor at Columbia (who is a professor at the med school and sits on the adcom at P&S), he was telling me that Harvard and Columbia are in trouble with the NIH because they've been keeping their MSTP students for too long (upwards of 8-9 years, more like 10-12). Obviously, you don't want to rush this training - you want to get a real PhD - but you also don't want them to keep you as free labor (NIH pays your tuition and stipend) forever. If any of you out there are in the Harvard or Columbia programs and it's not actually this way, I apologize in advance - but this is what I've heard.
My personal vote is for Wash U. Their research plant is phenomenal, and I think it's a great place to be doing research AND to get a medical education. It's supposed to be really supportive - every Wash Med grad or former faculty I've ever met has raved about the place. It was real funny to see faculty members at other schools I interviewed at start singing love songs (not literally) to Wash U during our interviews - most, but not all, were former students or faculty members. Feel free to PM me or post here if you want to talk about it more... I don't think you can go wrong either way.