Assuming the single rear wheel 350, it's mostly heavier rear suspension. Assuming you find two nice trucks for the same price, I would take the 350 so you have a little more capability, particularly if you considering a 5th wheel.
Bumper pulls won't see much difference because hitch weight usually isn't the limiting factor but 5th wheels are usually limited by pin weight and there the 350 will easily be rated for a bigger trailer.