Please dont compare subs with subjective interpretations that are incomprehensible for anyone else viewing the thread...
Why does the BM have better SQ?
What affects the perception of sound quality? For most its frequency response and distortion. The larger driver will undoubtedly have much lower distortion, but its frequency response is likely much different than the 10. Frequency response although highly specific to each vehicle/install (this is why subjective impressions of subs have no validity), can be manipulated through equalization to create the desired curve. Drivers with less capable displacement have limited dynamics and are much more sensitive to equalization...always go with more displacement if you can fit it.
Equalization can solve your FR tastes...
Exaggerated low-end? Done
Exaggerated top-end? Done
Flat resposne? Done
Never go with a smaller woofer if you can fit the bigger one. The larger woofer will do fine with the amplifier, they are much more efficient as it is. I would reckon that the BM (short throw, small diameter) produces a lot more distortion and has an exaggerated top-end (lack of low-end presence equates to the same thing).
I would personally recommend going with (2) TC Sounds 12" TC-1000s, in favor of a Mag or W7. Theyre on sale right now for $120 each and will easily satisfy