Am I?
2x BL or x1 BTL. I've only heard things like "Only if you have enough power to drive it's worth it" on the BTLs. Not sure on the BLs.
To make another reference, I had a guy interested in some of the L7s, he said his 4 JL 12W3s did much better at 1kW than the 2 L7s at 1kW, then again, during the way down here he blew his alt, so the amp was operating at some 10.5V. I blamed that. Makes a significant change, and the L7s have a lower sensitivity, would take more wattage to make the same as a lower wattage system with higher sensitivity, being said vaguely.
In logical sense for OP's question in my own personal opinion, the 18" BTL sounds superior to me if you're about to drive it with enough power, and enough box space.