Rob, I've found that it can be a toss up between the two. I'm a VB dealer, so I get a price break there, but there are still items I order from Moss or other sources. For instance, performance stuff usually comes from Winners Circle, APT, or Seven Enterprises. Sometimes the quality will be higher, or of a name brand from one supplier.
Until you get some experience in dealing with these people, it's kind of a crapshoot.
Having said that, I will say that W/C, Seven, Moss, and APT are all staffed with car people, whereas VB has "order takers" on the phone.
Jeff