Bull Bar attaches to lower points on the frame, from underneath the bumper. The WAAG Bar attaches higher up by your tow hooks. The Bull Bar takes away from your approach angle while the WAAG does not. The WAAG is also styled more like a push bar whereas the Bull Bar is made of tubing, which is mostly style, but will save you some damage in an accident.
Edit: There are a few different types of WAAG bars, I'm talking about the ones we get through the group buys.