Brian--
I am reading between the lines here, so forgive me if I miss the mark.
I believe that in Mexico one may not own "military" cartridges, so that eliminates .45acp and 9mm....correct? If i am mistaken, please forgive me.
So if you are looking at .357 Sig, then .40 S&W is also an option, yes?
Between the two, I would go .40S&W any day, mostly due to ammo cost/availability, and then because of noise if fired indoors (God help your ears if you discharge a .357 Sig without hearing protection, especially indoors).
Honestly, I have no use for either the 5.7 or the .357 Sig, but others will disagree.
But that is because I can own 9mm, .45acp. 10mm, etc.
And I do.
As far as which platform is the "best"....well, which one do you shoot the best? For me, that is the 1911, hands down. YMMV.
Best regards, Rich