I haven't tried the Wilcox, but just looking at them, the Wilcox mount has the Boa-dial suspension system and side straps, whereas the Nightcap is a basic 4-point harness design with a nape.
Hypothetically, this would make the Wilcox system more stable, since the sides length could be adjusted, and the whole system can be tightened under the back of your head. With a non-rigid system, a good suspension system like the Boa-dial could make a lot of difference, though again, I'm speaking only in hypotheticals here, I haven't tried the Wilcox system, though being made by Wilcox, I have no doubt it's an excellent system--this doesn't necessarily address the question of whether or not the cost premium is worth it.
FWIW, as you mentioned, the Wilcox does come with a shroud, and dial retention/suspension systems are typically more expensive than simple "H-nape" style harnesses. Beyond that, the Wilcox gives you more Velcro for the attachment of IFF devices and counterweights, which may or may not be important to you--but if it is, you would need to find a way to attach them to the Nightcap after the fact.
Were cost no object, I would probably go with the Wilcox for duals, but I would echo that unless you have a need for having a "low pro" suspension, I would almost always prefer to use a helmet rather than one of these caps, especially for duals.
~Augee