Wash U, but only because I am on the WL at W&L
But it all depends on what you want to do and where you want to be after you graduate. Wash U has a better national rep than W&L, but both schools will probably place the same in the areas you are looking at (ie if you apply to a NY firm, they are not going to admit someone over you, who is essentially tied with you for the same position, because of the school you end up choosing.
Both schools have a very different vibe though. W&L seems a it more relaxed, low key, small size, and in the middle of small town america. Wash U is somewhat larger, the students seem a bit more diverse and competative, and St. Louis is pretty big city.
Pick the school that fits you best, and if you don't know, go for the $$$