Thanks for your great question!
Schema.org has stated that using either is fine. Here is their longer form answer from their FAQ page on which to use:
"There is a general trend towards using 'https'
more widely, and you can already write 'https://schema.org'
in your structured data. Over time we will migrate the schema.org site itself towards using https:
as the default version of the site and our preferred form in examples. However 'http://schema.org' -based URLs in structured data markup will remain widely understood for the forseeable future and there should be no urgency about migrating existing data. This is a lengthy way of saying that both 'https://schema.org'
and 'http://schema.org'
are fine."
That being said...I think I'll go switch Moz.com to https now. Think they will keep http:// around for a while, though. Especially considering SEOs who implemented Schema before https and G doesn't want to penalize site owners for not knowing any better? <- My personal thoughts on this.
Thanks again!