Can't comment on Brussels, but have been to Antwerp plenty of times on the lash and have always had a great time. It compromised of drinking in about 3 Irish bars/red light for the weekend and creating your own craic, with little interaction with the locals. Avoid the shitty Turk kebab shops.
If its a stag, you won't need the locals. You'll bump into other Brits/Irish/yanks and have a great time. And the beer is top notch. I stuck to the draft Stella and its great.
I currently live in Antwerp and can vouch for that too Irish Paddy.
Way better than Brussels and way cheaper. 30 mins on direct train fromthe airport too...you cant go wrong.
PM me if you need pubs/ bars/ clubs list etc....cant help no end PocketRocket.