The question isn't really "G26 or G27" it's more of 9mm or .40 S&W. with that being said, I would go with the 27 and one of the big deciding factors is, you can have 3 different calibers in 1 gun. As stated above you can go 9mm with the change of a barrel and a mag(s). (With the way magazines are priced now might not be your cup of tea) or you can run .357 with just the change of the barrel.
If this gun is gonna be for self defense CCW or anything like that. I would recommend getting the gun you're most comfortable with. Don't get the 27 just cause u can have 3 calibers in 1 gun. But if you do only use it in .40 if you are carrying. Never use a conversion while CCW. But either way you go you will LOVE it.
"If You don't stand for somthing you will fall for anything"