That depends. If you intend to work on other people's code or currently-active projects (or anything likely to be active soon), starting with 2.7 and learning the differences later would be a good approach.
If you're just using it for its own sake, or only for building things locally that either don't need to run anywhere else or only need to run where you have enough control to install your own Python, and you don't and won't need dependencies that aren't currently ported... then definitely start with 3.x. :)
If you're just using it for its own sake, or only for building things locally that either don't need to run anywhere else or only need to run where you have enough control to install your own Python, and you don't and won't need dependencies that aren't currently ported... then definitely start with 3.x. :)