Amsterdam, hands down. There is so much to see, museums, history and the people are special. Dont forget you can see both cities since you are so close.

I've been to both and like both for different reasons.

We love Brussels for relaxing to do nothing, you can't beat Brussels in terms of value. Nice hotels at bargain prices and great restaurants with high end Paris quality food at a fraction of the price (for a song). Great food stores and beers.

We love Amsterdam for the special canal ambiance, Rembrant & Van Gogh art, the spontaneous & friendly Dutch people. Amsterdam hotels are expensive and we haven't found a single restaurant that we really enjoy even going with the local people (relatives).