@paulgottlieb Python 3 is generally the better choice, if you have the choice. Sometimes you have to use Python 2 because you're using some other component that only supports Python 2, such as Pythonista in this case.
But if you do have the choice, I strongly recommend Python 3, as it has far less special cases that exist only for backwards compatibility, like print being a special statement instead of a function, input/raw_input, bytes/str/unicode conversion, old-style and new-style classes, etc. If you're going to work with Python 2, you'll need to learn about these things at some point, but if you're just starting with Python I'd recommend Python 3 - it's a lot cleaner and easier to understand in a few cases.
The current version of Pythonista only supports Python 2, but a version with both Python 2 and 3 is currently in beta (if you're interested in testing it, see the forum thread on the topic). As far as I know it will be released as a separate app once it's finished.