I don't know which tweeter the SongTowers you are considering have, but my simple view is:
Songtower pros:
Transmission Line (quarterwave) bass - very nice for what it is (
7 5" drivers),but the flip side is it is not truly full range and most of us would use a subwoofer. The quality of the mid bass is still very good, but the true beauty of this bass is not used once you cross to a sub. Understand that other aspects are well designed (by Dennis),but I think the BMR is probably better for mids and highs. The Songtowers are ideally suited as a high quality 2.0 system for music!
MBR Pros:
RAAL ribbon tweeter (assuming Songtower has any other tweeter).
Phenominal dispersion - 30 degrees off axis tracks within 2dB of on-axis and 80 degrees tracks within 5 dB of on-axis - that is insane (see link below for details)
http://philharmonicaudio.com/BMR Philharmonitor.html