According to tests done by Tom Nousaine, the Servo-15 will go lower, but the Seismic 12 will output more SPL.
Have a look at the S&V review of the 40v3 system, including the Seismic-12, located here:
http://www.soundandvisionmag.com/art...&page_number=1
more specifically the tests results in pdf format:
http://www.soundandvisionmag.com/ass...2004145136.pdf
Also a HTF member maintains a page of Tom's results of Sub tests here:
http://members.cox.net/frankcarter/T...Sub%20Data.htm
FYI - I have had a Servo-15 for just less than a year now and love it. Have not heard the Seismic-12, but am very impressed with the low freq punch of the Servo. As Nathan says, the 15 is not small (H50 x W46 x D58cm, 41kg).
Seismic-12 is H37.5 x W36.2 x D36.2 cm, 30.4kg.
In the end for a higher price you can go with an extra db or so of gain and a smaller footprint with the Seismic-12, but I think you would probably be happier with the lower output of the Servo IMHO.