My recommend would be to go with 2.7, as it is compatible with almost all 3rd party libraries which is really where python becomes it's most useful. It's still under active development; so major updates in 3 are backported to 2.7. There's really no particular reason to go with three unless you want to work with straigh data bits instead of having everything encoded with unicode.
I use 2.7 and every other python professional I know does too