I recently acquired an FNP-45, my first FNH handgun. I use the flat backstrap because the arched one makes the grip just a bit too fat for my medium-size hands. I suspect if I had a smaller FNP (-9 or -40), I'd probably prefer the arched for their grips. I own a potload of SIGs, and I love the shape of their grip, which isn't all that different from an FNP-9 or FNP-40 with the arched backstrap.