Flying solo, I've found both to be about the same. BRU can have long passport lines. FRA can be a zillion-mile walk. Both can have miserable security.
However, flying with my wife or a guest, BRU has an added annoyance... the Premier/First/whatever security line has an automated gate instead of a human. It will let the premier pax through, but not their spouse/guest even if on the same reservation. There's also no way to get back out, and the queue dragons will send the denied person all the way to the friggin' end of the normal queue...
Agree with the posters who suggested ZRH or AMS, if you can find a decent routing...