There are incompatibilities between the 2 series and 3 series. At this point I think it's best to just go with Python 3. Some systems still depend on libraries that don't have a Python 3 version yet, but you are in the perfect position to avoid that, and when you do need those, you will have mostly transferable knowledge. But do pay attention that Python 2 and Python 3 are technically different languages, and are slightly but importantly incompatible one with another.