I'm an old fogey, interested in museums, architecture, and history, and not in nightlife, and I'd prefer Antwerp to Brussels as a base. I have good friends in Ghent (Gent) and so have day-tripped to both of those cities a number of times, as well as having stayed in each of them once. And, FWIW, my Gent friends agree.

Brussels definitely does have its good points, but they're more dispersed, and much of its coherence was diminished by unfortunate post-WWII urban development. Antwerp has more of an atmospheric area (over towards the river, not near the railway station, though the station itself is something to see!) and, for me, more to see. It's often mentioned as a great city for trendy shopping and clubbing--more so than Brussels--but, as I said, I'm too old fogeyish to judge.