I really like Brussels. Studied there for a year.
Never been to said festival, but if you like the line-up go for it I'd say (dEUS is nice). The problem is your visit is quite short and I don't know what kind of things you like. You'll have to plan ahead. If you want to see the European quarter (basically one of the biggest parts of the city); you can visit the European Parliament etc ... Or if you want to just get drunk or something you can also do that; loads of bars ... Dansaertstraat etc...
The problem you'll probably have is that Brussels is a bit of town where you have to know where to be (and where not). Gent is easier; 30 minutes train ride from there. You just walk around and everything is easy to find etc... They also have a festival; Gentse feesten 13/07 - 22/07.