If you will be writing any new programs in the Python programming language then check Lennart Regebro's presentation[1] and slides[2] on their compatibility issues. It is interesting to see the amount of changes they have made to make the language more robust and correct. This does mean that programs written in Python 2.x , to some extent, will be incompatible with Python 3.x so keep it in mind when deciding on which one to pick.

Links Used:
[1] Lennart Regebro's presentation - http://blip.tv/file/1949281

[2] Lennart Regebro's slides - http://liwo.polsl.pl/pycon-pl2008/materia142y/python-3-compatibility.pdf