In .40 S&W, one can select effective bullet weights in 155, 165, and 180gr weights. IMO, this is already an advantage, as variety is a good thing!
As far as the 357 Sig "besting" the .40 in terms of hard barrier penetration, here is some data directly from Winchester's 2002 online LE catalog for their Ranger ammunition.
{Steel}
125gr RA357SIGT....Pen: 23.4 inches/ Exp: .41/ Ret. Weight: 125grs.
180gr RA40T......Pen: 17"/ Exp: .52/ RW: 178grs.
165gr RA40TA....Pen: 20.4"/ Exp: .48/ RW: 165grs.
{Autoglass}
125gr RA357SIGT....Pen: 10.3"/ Exp: .49/ RW: 81grs.
180gr RA40T......Pen: 12"/ Exp: .61/ RW: 153grs.
165gr RA40TA....Pen: 11.3"/ Exp: .61/ RW: 139grs.
I personally consider performance with regards to service caliber handgun projectiles, to be measured in terms of penetration and expansion. Here is some other various data comparing the two calibers using Ranger and Federal Tactical bullets, which overall, are widely considered the best two designs available. The test's are courtesy of Doc Roberts over on Tactical Forums, and David Difabio of ammolab. All results were obtained using 4-layers of 14.oz denim, and are from pistols with barrel lengths between 4 and 4.5 inches.
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
Additionally, if one wanted "357 Sig like" performance from the .40SW, the new 135gr .40SW Corbon PowR'ball loading is fairly similar. The downside, is that it does not do well in barrier testing eg: autoglass and steel, so it would not make for an effective LE loading. However, in the 4-layer denim test, it has indeed performed well.
135gr Corbon PRB, from a Glock 22, through 4-layers of 14oz. denim. Reference source is ammolab. (Vel= 1311fps/ Pen= 14.1"/ Exp= .64).......
As shown, the .40SW can and does outperform the 357 Sig in barrier penetration tests and the 4-layer denim test, when using the better loads for each. While the 9mm and 357 Sig are on par with each other when utilizing the better loads for each.
IMO, one of the main reasons the 357 Sig has enjoyed some level of success, is that it is compatible with the .40SW. While it has garnered attention from some of in LE circles, it seems less civilian shooters are enamored with it. As was previously noted, Ruger, S&W, and Walther, do not even chamber guns for it. Additionally, HK never released a full-size USP chambered for it, and it has been said they will discontinue the compact version do to poor sales.
I like shooting the 357 Sig at the range, and it has proven itself quite effective in LE field use. However, it is my opinion that it offers nothing significant over any of the other available service calibers, and actually a bit less in some cases.
Best, jnb01