Do you actually need your bag overnight - or is just that you don't think you can check it in the night before?

Schiphol does have facilities to hold baggage overnight, and several of the airlines there do allow you to check your baggage the night before. I don't know if United does - this is something you will need to check. Staying at the Mercure seems pretty pointless if you have to go landside again in the morning just to check your bags.