You're probably right about Smart Update being used most often. And it seems the most foolproof and intuitive copy option, especially for someone familiar with clone backups. Heck, it's a big reason why I bought SD!
Yes, your description of modification time is good. I was questioning whether users would be aware of situations when modtime might unexpectedly change on certain files and adversely affect Copy Newer backups. Maybe that's likely to happen more often than usual with files on my systems because of how I create/modify/update them.
Thanks for the feedback!