To me, part of the difference is the case used in the 9x23 Win. I am limited as to what I can say about the 357 Sig, as I don't own one and, honestly, have never shot one. I do know a couple of people who have them, and they really like them a lot.
I do own a 9x23, and the case on these guys is pretty tough. I have to admit, again, that I have only shot this gun for a very short time, and may have to eat my words later.
Casey