If you want to wow people who don't know anything about medicine, go to stanford. Wash U may be more respected within some medical circles, but in the outside world, it's name doesn't hold the same weight as Stanford's name.
That way, if you decide during medical school that you want to go to business school or run for public office instead, you'll be in good shape. Stanford's name might also help you if you're an fp in private practice or something.