I answered to this before, but lately items on this forum seem to disapear
From a amount of classes point of view, both applications are about the same size (around 250 classes). I didn't have a deeper look at the AdventureBuilder, but since it's also a Sun demo app, I would expect it to have the same architectural features as petstore.
I guess the benefit of petstore is that there is more additional reading out there that's not only from Sun.
D.