There is a very good argument to be made for the .357 in a carbine, from a performance/cost perspective, IF
First, the .357 Sig cartridge case is based on the .40 S&W and uses a 9mm (.355") bullet. .40 S&W cases are as plentiful as dirt at most shooting ranges and are also easy/cheap to buy. 9mm bullets are in every handloading catalog and store that carries loading supplies.
When necked down to .357 Sig, the .40 S&W case ends up a hair shorter than a factory .357 Sig case. Normally, straight-walled, auto handgun cartridges (9mm, 40 S&W, 10mm, .45acp, etc.) headspace on the case mouth and brass that is too short tends to be inaccurate. HOWEVER, since the .357 Sig headspaces on the shoulder (and not the case mouth), this creates no problems.
Next, the .357 Sig represents a big jump in performance over most automatic handgun rounds (with the exception of the 10mm; however, brass for the 10mm is expensive to buy and almost impossible to get for free at the range). This exceptional performance is even further enhanced in a carbine. Here are tables showing the differences in velocity obtained in various barrel lengths: http://www.ballisticsbytheinch.com/357sig.html
For comparison, here are the tables for the 9mm Luger, which fires the same bullets as the .357 Sig: http://www.ballisticsbytheinch.com/9luger.html
Looking at and comparing both sets of tables, both cartridges pick up a good deal of velocity in the transition from a 4" handgun barrel to a 16" carbine barrel, as expected; however, due to it's greater powder capacity, the .357 Sig picks up an ADDITIONAL
200fps. This is significant, especially when you consider trajectory and energy at 100-150 yards.
Using carbine-length barrels also opens up possibilities for experimentation with the slower pistol powders.
So... you can readily get .40 S&W brass cheap (or free), .357 Sig loading dies are plentiful, and 9mm bullets are abundant and relatively inexpensive. If you handload, you can inexpensively provide yourself with lots of ammunition that will outperform your 9mm, 40 S&W or .45ACP, either in a handgun or a carbine.
Your .357 Sig carbine will shoot flatter and hit harder at longer ranges than the other cartridges, too.
Of course, if you don't handload, the economic gain certainly isn't there. For the non-handloader, the 9mm is probably the best bet for the money in an auto carbine.