Python in Browser: How to choose between Brython, PyPy.js, Skulpt and Transcrypt?

I'm very excited to see that it is now possible to code Python in the browser. These are the main candidates (please add any I may have overlooked):

But how to choose between them? The only obvious difference I can see is that Skulpt is based on Python 2, whereas Brython is based on Python 3.

Please note: This is not a request for recommendations or opinions. I'm seeking objective facts that would inform an educated choice.

This might be helpful too: http://stromberg.dnsalias.org/~strombrg/pybrowser/python-browser.html

It compares several Python-in-the-browser technologies.

From: stackoverflow.com/q/30155551