On my trip I spent two days in Hotel Saint Nicolas right in the heart of Brussels. They do good weekend deals being a business hotel and breakfast is more than a just a croissant! In Sint Niklaas where the Zythos festival is held I stayed in the Hotel New Flanders and it was fine. Good selction of bottled beers if you need a nightcap after the festival.
I agree with Beernuts friend by saying that its better to go to the Cantillon brewday in the morning when the going is easier. There's less pressure to get around the tour route and you can take your time, ask questions, and drink in the experience. Later in the morning it seemed like a madhouse. Of course this leaves you drinking various glasses of lambics in the early morning. Not a bad thing at all and if you're lucky you can grab yourself a table near the pot-bellied stove (its a cold place in March!) and order a bottle of your favourite lambic tipple nestled in a pouring basket.
Once the actual tour is over there's no rush to leave the premises and there's plenty of merchandise to buy too, including the carafe basket!
So if you're up early at the brewday, drop off your recently purchased collection of lambics at your hotel room then I'd recommend going to Sint Niklass for the beer festival. It's brilliant!