Either one is an excellent choice for carry. Have you shot either of them? The only real difference is that the 26 is 9mm and the 27 is .40. The 26 will hold 10 rounds (+ 1 in the chamber) and the 27 will hold 9+1 on the standard magazine. Everything else is pretty much the same. The 27 may have a little more recoil but I do not notice that big of a difference. If you can, shoot them both and see which one you like better. You won't be able to tell much of a difference from holding them since they both have the same dimensions.
I had a G26, very mild recoil, accurate, but I personally felt it was a bit small for carry. A better choice in 9x19mm might be a G19 if you don't mind IWB carry. Of course being a 45 fan I'm lovin' my G30, they carry as easily as the G19 and you get 10+1 rounds of 45acp to boot. Really, you can't go much wrong with any of em', Glocks are all pretty decent guns.
I have and carry a G26, and I truly appreciate the size, accuracy, and reliability of the small Glocks (my other Glock is a G30). As an added bonus, I got the economy of shooting 9mm to go with it!
As an alternative, if you want a more powerful cartridge, you could perhaps look into getting the G27 and then a conversion kit to take it down to 9mm...that way, you have the G27 in original form and caliber, and a drop-in barrel to take advantage of the cheaper 9mm ammo whenever it suits you, and swap out to original for the .40S&W whenever the mood strikes.
Like GaGal said, we have both. I would recommend shooting each one first. The 27 is a little snappier, but not bad. Either one is a good choice, but Happy made the second best suggestion with the conversion barrel. The best suggestion would be to get both.
The 27 is def more snappy than them all. Except the 10mil maybe. But never shot the 10. I have a 21 and 27 and thinking about getting rid of the 27 because I barely shoot it and I'm not too good with it. I kick ass with te 21 so I'm considering a 23,26 or 19. Don't know yet though.