A question about Python versions

[Gib Bogle]

I am learning Python, and using PyQt to develop a GUI that will be used to run a 
Fortran program on Windows, Linux and Mac OS X (I think Python is great, btw). 
Without thinking about it I downloaded and started working with a fairly recent 
Python version, 2.5.4.  I've now become aware of the existence of Python 3.1, 
which apparently is a major revision of the language.  Does it make sense to 
stick with Python 2.x at this point, or should I be starting off with 3.1?  If 
it is recommended to stick with version 2, should I use the latest (2.6.4 or 
2.7), and if so why?  Thanks.