They are both amazing offers. If this is strictly a financial decision then WashU is definitely the cheaper option. They are both comparable schools so you cannot go wrong with either one.
Also, just as a FYI - WashU has a PEP program where you can freeze all four years at current prices and you can pay it back up to 10 years. You can use tutition, room/board and fees towards it. You can decide how much you want to apply to the loan. The application is simple. You can cancel it at anytime or prepay it too. It is based on simple interest too. You can also get the loan as you go but it will increase every year. The tuition/room/board/fees usually increase about 4-5% a year so freezing it at the current rates is a smart option.