United is non-stop between Dulles and both Amsterdam and Brussels. It's only 7+ hours from here to Amsterdam, and 9 hours back, so the flights aren't particularly long. I find E+ to be fine for flights of that duration.
It might be cheaper to do a round trip rather than what amounts to two one-ways. The train between Brussels (or wherever you will be in Belgium) and Amsterdam is cheap and fast, so price out round trips from IAD-AMS and IAD-BRU and see if they are cheaper than IAD-AMS and then BRU-IAD.