@robin_bartolett -
At my institution, we started using Canvas before New quizzes. Therefore everyone here knew about Classic Quizzes. new Quizzes did not turn out to be an overall improvement over classic quizzes such that you would want to go to New quizzes only. By that I mean there are items in classic quizzes that are not available in new quizzes - in regards to settings(If I remember correctly - I still use classic quizzes, because they do what I need). New quizzes has some nice features that classic quizzes do not.
In all likelihood it will depend on the material being taught and what instructors want from the quizzes. My advice (other than the confusion factor) would be to make both available. Your job is tougher in trying to explain to instructors how to tell the differences and for them to figure out what those differences are - or which version they like. After a while, you may find that new quizzes will be sufficient.
Good Luck.
ron