I have lived in both cities (and stayed at both hotels). A quick comparison:

* Hotel: the Hilton Antwerpen is a much nicer hotel, but you need (a lot) more points for your 4 nights. If you have them, they are worth it. (Also, if you qualify for an upgrade, the Exec lounge in Antwerpen is nice. Brussel doesn't have a lounge.)
* Neigbourhoud: Hilton Antwerpen is ideally situated. You'll find plenty of nice bars within 5 minutes walking of the hotel. In Brussel, you'll probably need a taxi or public transport to get back to the hotel at night.
* City: I personally prefer Brussel, mainly because it's very international and it's the only real city in Belgium (IMHO). But if you want the "Belgian experience", you can get that in both cities.
* Day trips: the travel time to Brugge and Gent is about the same from both cities. So if staying in Antwerpen, you'll only have to count the extra travel time from/to the airport.
* Night life: I'm probably to old to be up-to-date, but I'd say that in Antwerpen the night life is a bit more accesible and easier to find from where the hotel is situated. In Brussel I have found the places to go to be more "spread out" over the city.

I have tried to be objective - but I'm afraid the conclusion is obvious...