Depends on what you really want. For someone going for the ultimate SQ, i.e. sealed box or IB install, conservative gains, etc..., they are spot on. If you are looking for loud with pretty good SQ, I would look elsewhere. If you weren't happy with the Assassins based on quantity of output, I don't think I would be looking at an SQ oriented sub. As far as a noted difference in quality of output, in the right enclosure, they would probably be distinguishable. In the anything else (ie the enclosure that might get you the output you're wanting) the difference would definitely not be worth the price difference especially considering that you would probably still want more output.
If the front stage is doing its job, the subs can really be considered an afterthought. You'll need them for full spectrum sound but they play such a minor role in the overall sound of the system. In the ideal setup they'll only be playing the bottom octave and a half of the audible spectrum. This is even less when considered as part of the normally used musical spectrum. If you really wanted to have your cake and eat it too, you could either do a small up front sub and then the SPL subs on the back or set up the front stage to play down to the 50ish Hz range comfortably and have multiple settings for the subs. Mellow for SQ and obnoxious for other times. There's a reason that Kicker L7s have been used in SQ title winning systems, and it isn't becuase they are hugely accurate...