Which Python to use?
tl;dr: Python 2.x is the status quo, Python 3.x is the shiny new thing.
If you're choosing a Python interpreter to use, I highly recommend you Use Python 2.7.x, unless you have a strong reason not to.
As more and more modules get ported over to Python3, the easier it will be for others to use it.
If you're starting work on a new Python module, I recommend you write it for Python 2.5 or 2.6, and add support for Python3 in a later iteration.