Lessee, we did this last summer, only we took a train from BRU. As I recall, the trains leave BRU every 20 minutes and go at an agonizingly slow rate to Midi, but still get there in max 30 minutes. Incidentally, this could well be faster than a taxi and it is WAY cheaper.

Then it depends on whether you actually HAVE your tickets for the Thalys already or bought them online from the SNCB site (like I did) and have to wait in line to get them. In that case, you have to find the one place in the entire station where you can wait in line and get the tickets. Took about 20 minutes from the time we reached the station, but the line wasn't long then.

So I'd say if you have to pick up tickets, book the 11:40. If you already have your tickets, go for it. If you have to wait, there are lots of places to eat at Midi.

Incidentally, don't buy online from the Eurostar site, the SNCB site is way cheaper on a round trip especially.