I use both. AD has admittedly way better capabilities and sounds über fantastic. I just can't figure it out. I can't get it to do what I want probably entirely based on lack of knowledge.
With Session Drummer 2 I get what I want fast and have the controls easily understood and it sounds very good.
I hope to learn AD as I become more familiar with it, but it really can be frustrating. That said, it has fantastic sounding kits and I wish I could control them.