Steve, you can't possibly give an opinion between the 2 of them, you have admitidly never used a Nightshade.
I have, I have also used a old XXX and new XXX, The nightshade is a slightly different beast, the XXX is a little more SQ, however the Nightshade is no slouch either, no where near the "fartbox" sounding sub in your typical system. they can sound very nice in a 32-35hz box, and get stupid loud doing it.
Either way the OP will be happy, I have no doubt about that. But if he want to get loud and sound good doing it, I personally would go with the NS.
OP there is a reason I am a Sundown dealer, the service is top notch, and the products are just as good. I have been a dealer for car audio from the early 90's and I have never had a manufacture that deals with me the dealer and you the consumer the way Sundown does.
It really doesn't matter what you get, you will be happy, you have picked 2 awesome subs, and there ar many more you could add to the list. You asked for opinions, you got them, steve wants the XXX and others seem to suggest the SD. Shat are you going to do?