The Jl's will be capable of better SPL by far, however will lack in some areas of SQ. The perfect on the other hand is quite possibly one of the best SQ subs on the market today. Clear, well defined, and totally accurate bass response from them. The Jl's tend to get a little muddy sounding arround the 80-120hz range however still produce well.
It may be hard to tell from a simple trial, however a true SQ enthusiast will be able to determine quite a difference in the Perfects SQ from the JL. -After all, in SQ- everything has to be precise and clear.
I have spent many years as a active SQ competitor and have placed 2nd in a locally sanctioned USAC event. At the time I was using 2 Jl 12W6's, however a year and a half ago switched to 2 Perfect 12.1's. The difference was incredible. Mind you the Jl's sounded very nice, however, the Perfects were much more impressive. In Both applications the subs were flush mounted facing up in the rear deck of my 1985 Ford Thunderbird.
Sadly though, as I was preparing for key IASCA SQ event this past july; my tbird was claimed by a large warehouse fire where it was being stored. While the car is lost, my policy is the same with my other vehicles as it was in the t-bird. I will openly demo the vehicle to anyone who wishes to hear it. Those that heard my system both ways, even the ones who didnt know anything about car audio, but just liked music, all agreed that the Perfcts were a lot more defined and detailed.
The Perfects would honestly be the better of the two.
Take it easy,
-zane