I've recently completed a project of moderate complexity. I began development on Python 3 in a Jessie vm, and later ported to and deployed on Python 2.6 on CentOS 6.

The process was not without its challenges, but developing on a modern platform and backporting was a breeze, and I'm not worried about upgrades.

Python 3 for all intents and purposes, unless you're in a field where Python 2 is still prolific (but if you are, then you probably already know that).