If you don't have the option of going to both places it's usually best to follow the crowds unless you have some special interests or deliberately want to go off the beaten track. I mean, there must be a reason why Paris is the world's most popular (or 2nd or 3rd) tourist destination. As for doing both, a train from Paris to Amsterdam takes between 3 and 5 hours for the fastest connection. Or you can fly. None of that will be cheap, but what in a western European metropolis is? Whatever you do in this area, just avoid Brussels.
=== EDIT. Bummer. Thread from March this year. Guess you've long gone (and returned). Never mind... ====