Send emails through the new email program to take advantage of new features.
Continue sending emails based on leads' time zones.
Sending emails at different times based on time zone requires either multiple flow steps in one smart campaign, one flow step in multiple smart campaigns (this is how we had been doing it), or multiple email programs.
If using multiple email programs then it sounds like multiple emails will need to be created which makes email performance reports look sloppy as what is essentially the same email is returned in multiple lines instead of just one. Multiple emails for what is the same email also makes using "Was Sent Email" filters more difficult as well.