The difference is that the two don't interoperate. You can't import a Python 3 module from Python 2 or vice versa; you have to use completely separate interpreters to run them.

I'm suggesting a model in which one interpreter runs both Python 2 and Python 3, and the underlying types are the same, so you can pass them between the two. You'd have to know that "foo" created in Python 2 is the equivalent of b"foo" created in Python 3, but that's easy enough to deal with.