You can go to XLN website and preview all the ADPaks. Just click on the Pak that you'd like to hear and there are 3 MP3 files you can listen to. I too ended up getting the Fairfax kit. It was a toss up between that and the Funk Pak. The Funk Pak has some really nice sounding drums, but so does the Fairfax.... :)
The kits that come with AD2 also sound really good. I had already bought the Jazz collection (sticks and brushes) for AD1 a while back...and I'm sure the new kits for AD2 will probably be even better.
I also have several of the kits (including the Jazz kit) for Superior Drummer 2.0. And have to admit that I have preferred the sounds of SD2...but I haven't had a chance to load up the new kits for AD2 yet. Will do that tomorrow when I get to the studio.