The Brahma is still the best high-linear-displacement sub I have ever heard, but I must qualify this by saying I have yet to hear a Mag. Personally, I would buy the Brahma based on the 4 that I have heard, while considering cost at the same time; keep in mind that Kyle from
http://www.acoustic-visions.com is a very nice guy and should be able to take care of you.
Again, not saying anything about the Mag because, of the two, I have only heard the Brahma.
I'm sure as the primary engineer behind the designs he would have a lot to say about it. Coil diameter is valuable only in that it is a contributing factor to the surface area to mass ratio of the voice coil. I think it's rather intuitive to say that a voice coil with a higher surface area to mass ratio than another is capable of handling more power (without considering other cooling conventions). Don't see how there is too much more to discuss in that regard.
Neil