I would lean towards timbrally matching the other speakers in your system if possible, which usually means at least using a speaker from the same manufacturer. Failing that, I'd try to pick a speaker that has similar specs to those already in your system (range, crossover and sensitivity). I'm not at all certain that I'd trot out to buy a Bose speaker for a rear surround, but if you found one for free it's worth hooking it up to see if you like it.
Bryan...did you look at Parts Express for flush mounts?...