When I bought, I bought the .40 Sig. Later I picked up a .357 Sig barrel. Both are great shooters, I am please with accuracy and function. With the Sig, buy either one, then purchase the other barrel (about $125.). Nothing else to buy, it's just a drop in and you are ready to go. I shot the .357 the first time when qualifying for renewal of my CCW. For out of the box ,and to the range with no practice shots, I was exceptionally well pleased.
The Texas DPS went from the .40 to the .357 Sig several years ago. Their "experts" seem to think the .357 is a better overall performer. I don't think you would go wrong with either one..............