If the barrel is not touching the stock, the difference is more in technique than anything. Its all physics, you have energy moving forward and backward, you have tension and torsion against the stock and bipod by the shooter, also muzzle rise. The goal is to find a combination of muscle control to counter the recoil in the exact way every time, this is where bipod loading becomes a necessity.
Shooting off of bags, the rifle "rests" in pretty much the same way every time, the shooter may use his support hand under the rifle buttstock, or may put a hand on the forearm,(to counter muzzle rise) as long as its consistant. There is less issue with vertical stringing off of bags (vs inconsistant bipod loading).
Bipods are usually favored because of practicality in the field.