Couple of things to consider.
A body lift 's only purpose in life is to get you fender clearance for larger tires. It also raises the center of gravity of the truck making it a little more tippy.
A 6" suspension lift may yield a little more flex then stock, but if you keep the stock radius arms you are not going to gain much articulation. However if you get the 6" suspension lift with the extended arms, brake lines, shocks, etc you will then get the benefits of added articulation.
Larger tires = more ground clearance, a larger contact patch on the earth and more traction.
Personally I would rather steer clear of the body lift, go straight for a 6" suspension lift (Skyjacker) and trim the fenders or run fiberglass to clear larger tires. This way you get the best of both worlds, larger tires and more articulation.
Yet again it is all a matter of what you want to use the truck for. Looks = BL, off road ability = suspension.