Since it will take a while for the 3rd party libraries to be ported to 3.x, start with 2.6. The versions are not different enough for it to be a big problem either way you learn them - but there are more books out for 2.x - like the Python in a Nutshell book, which I find to be the best reference (though not the best book for beginner programmers). Then, you can refer to the "What's New in Version x" entries in the documentation on the Python site to learn the later versions.