Cool question. I am going to give you a short answer right now to get this topic rolling. I have a gig tonight so I can't spend too much time right now.
I know that there are several branches of Bagua out there. The ones that I hear of the most are Yin or Yinfu style, Cheng or Cheng Ting Hua style, and Gao style.
I currently study the YiZong Branch of the Gao Yi Sheng style Bagua. There is a large emphasis on long body postures and deep stance work. Gao style is also charachterized by it's 64 linear Post Heaven or Hou Tian forms. These 64 forms primarily serve to train the body as well as the fighting principles inherent within them. It is a very comprehensive system.
From what I have heard and from what I have seen, Yin style has more emphasis on fajing style hitting in its forms.
As far as Cheng style is concerned, its circle walking forms or palm changes look similar to those that I have seen within the Tianjin branch of Gao style Bagua.
I think that the similarities are there because Cheng Ting Hua is a bit back in the Gao style lineage.
Dong Hai Chuan (the founder of Baugua) had two top students, Yin Fu and Cheng Ting Hua. It is from these two guys that most of the other branches of Bagua have been developed.
I hope this gets the ball rolling a bit.
Time to go