If you have enough time why not go to both.

I was in both cities in August. I liked Brussels a lot - toured the palace, bought lots of chocolate, enjoyed the Grand Platz and of course went to see Mannequin Pis. Pretty nice cathedral there too.

Brugge is not a far drive. It was quaint and charming, but also pretty touristy. Kind of reminded us of Carmel in that sense. You can get a pretty good sense of both cities by spending just a couple of days in each.