IMO Steam is less harsh, but you have to be damn quick getting the bow to the form, unless you build a steam chest around it.
Heat gun is better for in situ use, e.g clamp it up on get it partially clamped , heat the desired area and tighten the clamps or pull it the rest of the way.
Both work, both need some time for the wood to stabilise afterwards.
To me the big advantage of dry heat is you can combine bending and tempering/heat treating into one operation.
Del