V3 - SmartApps vs Automations— How do you decide which to use?

If it was a pain, maybe you should have waited for your migration notice so that this was automatically done? Anyway, if you go through the official migration, they get created with the automation creator.
There are currently some limitations compared to Routines in Classic, though. Namely using a mode or device as both a condition and action. And also no way to set a cotnact sensor says open for X time condition. You can read more here.