[Robe]

Definitely arched for me. The arched housing fits the shap of my hand better as I grasp the gun. With a flat housing, I had trouble engaging the grip safety with a proper grip when doing fast draws. The arched housing somehow helps my hand to find the proper place and helps me to engag the grip safety.
The wedge is not the same. The heel of the wedge shape digs into my hand too much and cause real problems in prolonged shooting like in a class.