I have both Python 2.7 & 3.1 installed on my Ubuntu box :)
The major difference that between these two versions is Python 3 has limited library support and the fact that most current Linux distributions has Python 2.6 - 2.7 as default.
If you are just in learning phase of the language I think any of these versions should go smooth.
“Knowing is not enough; we must apply. Willing is not enough: we must do.”
- Bruce Lee