Personall, I would avoid cabs at peak times of the day - Brussels is congestion city. I've done the run to the airport (admittedly from Le M) in 25min & 30Eur. Other times it's well over an hour. I don't know what time Brussels rush hour starts so 630 could be sticky.
The train however which goes from Midi I think is about 25min always :-) and I think about 7Eur. Or, the safest but dullest bet is stay at the Sheraton at the airport.