Keltec 3AT is a .380. Isn't the Glock 26 a 9mm? If you find them to be equivelent in your shorts, then go with the 9mm.
The Keltec is very light weight, and I find it to be accurate only at close distances. Its an inexpensive gun and I rely on mine as a back-up (as in when I can't take something bigger with me).
My Keltec has been 100% reliable.
Still, if you want a primary gun with more oomph and you can conceal the G26 with no problms (I think it is a fatter gun and heavier and holds more rounds and carries a more potent round), then go with the Glock.
You already mentioned about the Sig: It's like the Glock, but its a .380 round instead of a 9mm.