Most systems install 2.7 as a default but everything is quickly moving to 3. It really isn't an either/or in terms of learning though. Basic python is the same. The differences come in with all of the third-party toolsets and libraries available in 3. For example, I am working with Machine Learning and the Tensorflow and SCI-Learn libraries require 3. So if you are doing general admin work, you might ask around about the third-party components you will be working on, and if 3 is a requirement, go for that. Either way, when you are getting started, you will waste no time learning on 2.7. The language is the same.