I haven't done a bus or taxi transfer from BRU to Antwerp, but have done it via train. It's pretty easy and inexpensive, but probably not the quickest route if you have a meeting, or particularly good for someone with a lot of luggage. Hope this helps anyway...

Trains leave BRU for Brussels city center (Nord or Central stations) every 15 minutes, and the ride takes about 20 minutes (3 Euros one way). Trains leave from Brussels-Nord or Centraal twice an hour for the 40 minute ride to Antwerp (about 20 Euros round trip). The Hilton Antwerp is at the Grand Place (main square), accessible by either taxi from the Antwerp train station (it's under 3 kilometers away), or a quick ride westward toward the river on their tram/metro system (2 Euros).