Should we start learning Python2 or Python 3 now?

I guess they are pretty much similar?
It shouldn't be a problem to adapt to py3 later on

As long as you are not using any advanced python, there isn't much difference, but you should definitely try to stick to PY3. VFX is one of the only lazy domains, that didn't switch to Python3 yet. Officially Python2 will be deprecated (not be supported) from 1.Jan 2020.