Add to the cost of a Shield the cost of a better trigger and night sights and then consider your personal needs. If you want to pocket carry, and I don't know that either the Shield or the P365 is optimal for pocket carry, the reduced size with added capacity of the P365 may be worth a couple hundred to you. That and I want to get reports on shootability from some besides the professionals, but it sounds like the Sig is a far better shooter for most.
I am not saying that the Shield is not a fine gun, for what it offers. I like that it can be had with or without a thumb safety. One thing you can't get with it is any kind of rail. Many complain that the SIG rail is proprietary, but at least it has a rail.
Then you have the ability to change the grip. It isn't clear to me just what kind of grips will ultimately be offered.
As for Shield adding a higher capacity magazine, the Sig already has a 12 round mag available. That is 13 rounds.
I don't know if I would want to shoot it out of such a small gun as either the 365 or the Shield, but is the Shield rated for +P?