The problem with numerical size differences is that they are somewhat deceptive in the real world. A quarter to a half inch here and there really isnt anything between similar models, especially if your using a good holster, but even if you dont. Short of wearing a "fitted" or snug shirt, I can, and do, easily hide my basically full size P229 and a double reload under a tee shirt, or another light untucked type shirt, all the time, so how does the minor difference between the smaller guns really matter? Using a proper holster, you should easily be able to hide either of them without issue. Even between the "full" and "compact" SIG's, and many other models, the only difference on paper is again, that quarter to half inch, and when you compare them in your hand or wear them, it really means nothing.
Unless your specifically looking for a heavier gun, why compare the SS version of the SIG? The alloy version is more desirable in that respect, especially if your carrying it in a place other than on your belt. Its not SIG's fault that Walther doesnt offer a lighter weight gun and something maybe Walther should address. I have to add that to Walthers shortcomings, not penalize SIG for it.