The BTL is the better option as it allows for the customization necessary to do what you wish it to do. The MT has a much tighter gap than the daily BTL so in that situation, it might make for a little more SPL, especially if you start adding cooling options to the BTL and take even more steel out of the motor. But...if you go with an SPL BTL, add 3rd magnet and pole chamfer, you'll likely be scoring higher than you would with the MT. Naturally, enclosure and listening environment have pretty big effect on your results.
I mean honestly, if you design sub A, would you really make a sub B (the flagship of your new company) with less potential than your first sub?