Having stayed at both, I'd highly recommend Brussels as a base for short trips if you can manage it. Ghent really lends itself to a few days of walking around soaking up the lovely buildings and very nice restaurants. Rushing off to catch a train every day strikes me as out of sync with its character. The Brussels Marriott, on the other hand, is centrally located near train stations as the already mentioned and just perfect for either day trips or exploring Brussels. Lots to do and see within walking distance.

Ditto what the OP said about the Renaissance - too far off the beaten track.

There's also a Courtyard about halfway between the airport and downtown. I have no info about it.

I'd call the GM in Brussels and ask sweetly...good luck!