I got a Mann from Rockauto pretty cheap. The quality seems to be just as good, if not better than the Denso it replaced. In terms of air filtration, the major brands seem to be on par with each other for the most part.
I guess the 4.0 isn't as picky as the Duramax; I've been sticking to AC Delco for the Van (all the expendables are all manufactured by a heavy equipment speciality company).