Welcome to Python 101 at PyOhio 2013!
Python 2.x is the status quo, Python 3.x is the present and future of the language
I picked Python 2.x as it is what I am most familiar with. I have yet to really pick up and play around with Python 3. It is still the "norm" and most libraries you'll come across are in Python 2.x, and many of those have already been ported to Python 3. The differences between Python 2 and 3 for the most of what will be covered in this tutorial are small changes, and a list of what is different can be found here.
Most *nix systems come with Python 2.x and Python 3.x (unless you are running Arch Linux) with Python 2.x being the default (Arch Linux defaults to Python 3.)