Bottlenecking a .40 down to a 9mm, is just making a 9mm +P or +P+ rd.
Ok here is my un-educated 2 cents.
.40 S&W is generally far more available then .357SIG and is usually cheaper by an amount similar to .40 vs. .45.
.357SIG is a very very very hot 9mm for lack of a better term. Now because of this you can find some 9mm +P+ that basically performs like some of the .357SIG offerings. The best .357SIG will perform better than a hot 9mm though, IMO.
.357SIG is loud, has lots of flash and a snappier more upward kick then .40. This is of course in my opinion since recoil is very subjective. It is by no means harsh through a SIG P239, just snappier.
.357SIG has been known to batter even the best guns and puts more wear and tear.
I would THINK .357SIG would be a better barrier penetrator then .40 but I worry, probably needlessly so, that .357SIG may over penetrate in situations that you may not want it to. I have heard this really comes down to the design of the hollow point and some have great reputations and some don't.
If I were you I would get the gun in .40 and convert to .357 at your leisure. I personally love .357SIG for fun but carry 165 grain .40 caliber ammunition when I carry my P239. I am sort of a bigger hole is better kind of guy. Not that I want to start any caliber wars as I frequently carry 9mm, .357 Magnum, .40, .45 and I believe that with proper ammunition all are more than adequate in the self defense role. I am just saying that I feel better with bigger bullets making bigger holes as a psychological thing for me, hence my most carried SD gun is a SIG P245 in .45 ACP.
So in short for me, .357SIG is fun but I prefer .40 for real work.
Federal Tactical 125gr 357 Sig: Vel= 1390fps/ Pen= 13.5"/ Exp= .62
Federal Tactical 180gr .40SW: Vel= 1000fps/ Pen= 12.4"/ Exp= .73
Federal Tactical 165gr .40SW: Vel= 1013fps/ Pen= 12.9"/ Exp= .65
Ranger 125gr RA357T: Vel= 1340fps/ Pen= 13.5"/ Exp= .63
Ranger 180gr RA40T: Vel= 910fps/ Pen= 14.2"/ Exp= .65
Ranger 165gr RA40TA: Vel= 1166fps/ Pen= 13.5"/ Exp= .66
For the sake of comparison, here is the data for the 127gr +P+ RA9TA through 4-layers of 14.oz denim, fired from a G19. Test is courtesy of ammolab.
Ranger 127gr +P+ RA9TA: Vel= 1219fps/ Pen= 14.8"/ Exp= .63
http://www.thehighroad.org/archive/index.php/t-56038.html*****
I wouldn't mind getting a .357 Sig barrel for my M+P full size .40... Just a barrel swap as the mags cover both. Like the .45GAP, it works fine, but you'll pay extra, or look harder to find it.