I've owned an SE, SX, and L7.
Here's how I would break them down.
SE=lower power applications, good sound. Not very loud though
SX=little more power, upwards of 1kw, better SPL
L7=midrange power in a daily driver application, more cone area than its round counterpart, should get just as loud, if not louder, than the SX.
I particularly didn't care for my SX. I think I used it a grand total of 2 hours. It got its *** handed to it by the type-r and pretty much every other sub I've ran since then, including the L7.