A lot of users prefer keyboard to a mouse. Both widgets allow that. However, Chosen handles tab better. With Chosen, tab completes the entry and moves to the next form field. With Select2, I could not get tab to move to the next field. Select2 messes with the tabindex property. I am guessing that is part of the problem.
This is the reason I used Select2 instead of Chosen. Tagging allows the user to input something that is not already in the select widget.