So I have a two piece setup (RS Taichi perforated leather jacket, Vanson perforated technical pant) that I took to a leatherworker to add a heavy duty, high quality zipper so they can be attached to one another.
This setup works great.
The jacket has all the features I want, as to the pants, and the jacket also has pockets that will hold my keys, wallet, cell phone (Nexus 5, i.e. decent size) etc. Being zipped together I have all the security/safety of a one piece, but with the obvious advantage of being able to take my jacket off (or occasionally decide to not wear the leather pants).
I also use an amaaaaaazing wallet - "Slim Fold Micro" which is a wallet made of Tyvek (the sturdy stuff of FedEx envelopes). This keeps my wallet at almost 50% the thickness of what it would otherwise be.
I have to say that Maxpedition femur bag looks pretty sweet for days when I need to haul "stuff" for whatever reason, and is probably more comfortable than a backpack or fanny pack.
EDIT: This bag looks even more practical, and has a lower profile. I might get one and report back on it:
ThinkGeek :: Grab-It Pack Gadget Holster