Glockfan35, I'm not sure about the legal aspects of what some others are talking about, but they may very well be right. But from an objective "ballistics" point of view..........
......I mentioned a G20 to you, but it was more in humor than anything else. The 10mm round is over-kill for SD purposes. When it comes to the power of a handgun, more is not always better, and the 10mm round has way more power than is wise for self-defense purposes........ and way more than is needed for the vast majority of LE purposes.
The more traditional calibers (9mm, .40, .45..... and even .357 sig) are good at what they do because they occupy that "sweet spot" between too much - and not enough. The ballistic performance of a 10mm pistol round more closely resembles that of 44 Magnum than it does the traditional SD/LE calibers. A round that is more at home bringing down a charging 800 lb bear, does not make for a wise choice when it comes to self defense against a relatively fragile human predator.
It's best to pick the right tool for the job, and for the job of self defense (in the vicinity of any populated area), 10mm isn't the right tool. It's a VERY powerful pistol cartridge that is put to better use in wide open spaces, against much more robust targets.
Your 9mm G26 is a fine year around carry gun/caliber. If you feel like just owning a 10mm for the fun of it or if you'll be tramping around out in the Rocky Mt wilderness, then a 10mm makes sense. I've been thinking of getting a G20 for quite while now, but I'd never even consider using it as a carry round - at least not if I have other guns/calibers that are better suited for that purpose, which I do. I just want to have a 10mm pistol for shooting fun, and possibly for the occasional hiking trip out west.
As far as .357 sig? It'll work well, much like 9mm, .40 S&W and .45acp does. I'm not sure I'd bother with it though. Your G26 is a good year around SD package.