I'm using the Hadley Pro, but it is definitely less comfortable for bigger cameras and lenses.When I bought it, I think the "One" din't exist yet.
So, if you own an EOS 5 or 7 series camera , I would't hesitate and go for the wider model. Quality is superb (I own 3 Billinghams!).
I own a 6D. I started with a Billingham Eventer, which I find superb, but it is too large. I really do not mind the width and the height, but it is too deep (front to back). Then I changed for the Hadley Pro, which I like, but it is really tight for my lenses (I use a Canon 24-70 2.8, a 70-200 2.8 and a 50mm and a TC2x). So, I thought the Hadley One would be right in the middle of the Pro and the Eventer, however, these bags are not inexpensive, so I am trying to find someone with a kit similiar to mine who was already tryed it on the same bag.