Shadow Systems started out selling aftermarket upgrades, now they're selling essentially aftermarket Glocks. They have a generally good reputation, and a new Shadow Systems gun is cheaper than a similarly upgraded Glock. Their triggers require some break in by the user, slide tends to hang up if you pin the trigger. Manual suggests a two hundred round break in period, but I think a large part of that can be done by dry fire. Regarding warranty Shadow Systems has a lifetime warranty for the original buyer, contrasting nicely with Glock's one year. Either gun is easy enough to just replace broken parts on. Most Glock holsters should fit the Shadow Systems guns without issue. Aftermarket Glock parts often do as well, though there is little need to modify the gun.
The SIG P365 is a completely different class of well regarded guns. Good holster support as the P365 is a market leader. Aftermarket support is similar to the the comparable Glock pistols. Warranty is a limited lifetime, officially with a number of ways to be voided if you're dumb enough to tell SIG you did something dumb with the gun.
Either gun should serve you well. If at all possible, shoot both before making a purchase. The size difference between the guns is probably the more important factor. If you are attempting to conceal the gun you will find the P365 very easy. Assuming you are an average height and weight male you'll have to put more effort into the Shadow Systems, but it can be done. Again, both good guns.
