In a SQL install, which would be you wanting to get loud AND sound good, the W7 is the winner. It has more linear excursion than a Solo-X does, and performs better in enclosures that are designed to do so (sealed, low tuned ported boxes). In that application, the W7 would usually get louder and sound much better. In that application a Solo-X is an extremely inefficient sub that sounds horrible.
In a dedicated SPL install, with substantial power (read: high tuned ported or bandpass enclosure, with lots and lots of power for burps), the Solo-X has more potential because it is inclined for this.
i think the w7's are great subs but a little pricy solos are much better ( i am a fan of squares ) I wish i could afford them i got a re sx 15 in my trunk i like it alot