Backpack for me.
Here in Montana (I assume Colorado would be the same) my mornings leaving camp might be 20 degrees and by mid afternoon it's 60 degrees, so it seems like I am constantly taking layers on and off, carrying lunch, water, knife, saw, survival type equipment, etc. When I leave camp in the morning, I don't intend on going back until that evening and can't seem to get all I need (want) in a fanny pack.
I have no trouble shooting with my Bison Gear pack on, but others have stated and I have time, I'll drop my pack when setting up for elk coming in. That way you can maneuver around easier to get a shot if necessary.
Whip has a great selection of packs, many of which would serve you nicely.
My $.02's
Ron