well do u mean sbl(dont know what this is) or sql or spl. Assuming that u want spl since u said "whats louder" I would have to say that they would be very close to each other. a 15" l7 would probably be able to beat a 13.5"w7 but not by much and thats even if it could. But since you just want loud you can get the l7 for about 1/4 the price of the w7. As for which is the loudest sub on the market thats a tough one there are so many names out there. AmericanBass VFL, RE MT, DD 9917, Solo X, all these subs can handle 5krms+ and some can handle 15k burps. If you really want loud u are gonna have to do a overhaul of your electrical system and have several thousand watts.
It depends on what type of SPL you're trying to accomplish. If you're comparing sealed box to sealed box, the W7 would outdo the L7 in both SQ and SPL. If you're strict SPL, using ported or bandpass enclosures and packing power on the drivers, the L7 would win out. Ultimately, yes, the L7 can get louder if you're talking SPL competition with ported boxes tuned to the resonant freq of the cabin, but it's a moot point for someone who plans to use a subwoofer for everyday SQL. That's not to say that an L7 couldn't function well in a SQL application, it just tends to suffer the side effects of a SPL based woofer.