For a carry weapon I would probably lean toward the 9mm, but for a nightstand weapon I would go with the 357. Like several have mentioned, the cost per round differential is substantial. What you choose will affect your wallet.
I have two Sigs, one was born .40 and has a bar-sto .357Sig barrel. The other was born .357Sig and I just picked up a 9mm barrel for it. This let's me have the best of both worlds between cost and on-paper ballistics.