What are the Pro's and Con's of Auto-trac vs. viscous coupled AWD?

I like the idea of AWD in my Silverado SS, but sometimes I wish I could switch to 2WD.

 

You just said it, the ability to switch to 2wd. I would think that the 4wd would be better for traction at the track, but there are a few SSS here that have cut some KILLER 60 foots..... I guess one benifit would be a 4wd can be dynoed much easier that a all wheel drive......