I use both depending on who in my department I am helping (Some people prefer 2.7, others 3.5). Anyway, I use Anaconda and my default installation is 3.5. I use environments for other versions of python, packages, etc.. So for example, when I wanted to start using python 2.7 I ran:
conda create -n Python27 python=2.7
This creates a new environment named Python27 and installs Python version 2.7. You can add arguments to that line for installing other packages by default or just start from scratch. The environment will automatically activate, to deactivate simply type
deactivate (windows) or
source deactivate (linux, osx) in the command line. To activate in the future type
activate Python27 (windows) or
source activate Python27 (linux, osx). I would recommend reading the documentation for Managing Environments in Anaconda, if you choose to take that route.
conda version 4.6 you can now use
conda activate and
conda deactivate. The use of
source is now deprecated and will eventually be removed.