I plan to install Python and Selenium on MacOS 10.12.1. 
What would be the better choice - Python 2.7 or 3.6.5? On windows I use 2.7, because I read that it works faster. But are there any proven obstacles while working with Python 3 and Selenium on Mac?
I have been using Python 3 for quite a long time now. There is not really a performance issue that is quite noticeable. I would suggest Python 3 or which ever is the newer one because as they keep developing they will be adding new things that you might need and remove some older vulnerabilities. I would suggest you upgrade to Python 3. There might be some getting used to the new syntax but it can be quickly come over as you keep developing.
MacOS Sierra 10.12.1 - selenium - which version of Python?