which version should I download?

Answer 1

you can download any 3.x version

Answer 2

3.0
though 2.7 and 3.0 are both common

Answer 3

If you want to use python with QGIS or Blender or any other python integrated they mostly use 2.7 and that's mostly why they are keeping it alive I think. And with the 3.x you get new update of some packages but that's pretty mutch the same. I'm using both and it's not a big deal.

For a beginner I would prefer the 3.x.