Well, the 27 will fit all your 26 holsters, personally I would go 27 before 36 but the 36 does give you something a little different, a little bigger than your 26 and smaller and lighter than your 1911.
As far as cartridges go, I personally kind of like .40 better. And my 27 has been great, really won me over to the Glock-side. Especially after I put a smooth faced trigger in it.