The XV15 would definitely be worth the extra coin...deeper extension, better build quality, and about 99% sure more output. I believe the SW-115 is rated 121db @ 30hz 1m 1/8 space corner loaded. Its hard to make an exact comparison but I believe you would subtract 15db for 2m rms numbers for comparing with data-bass.com/systems.
Aside from that there were a couple members over at avs that stated the SW subs did not sound that great. I have not herd one first hand so thats all I have.