If it's a decent red light district you're looking for, the one in Amsterdam still is the most "luscious" as far as I know.
As for Belgium, the one in Brussels is dirty as hell: tons of Albanian girls getting pimped against their will by lots of Albanian pimps and loads of Moroccan hoodlums hanging around. Id stay away from that one.
The Antwerp one is pretty tourist-y. There's a famous dance café smack in the middle of the red light district lol. You can have a good laugh when you take a stroll through that district on an evening. Stay out of some of the bars in the sidestreet of the district, unless you like to have 5 bucks ripped from you by illegal Nigerian prostitutes.
The one in Ghent is small, but clean.

Don't ask me how I know all this lol.
Then again, I guess you're not taking a trip to Belgium or Holland solely for the red light districts