Question
How are Terraform provisioners different from Terraform User Data? Both usually involve scripts to customize a server.
Answer
Provisioners have built-in support for CM tools and can have scripts longer than 16 KB; these things are not true with User Data. (This was taken from page 213 of Terraform: Up & Running, 2nd Edition by Yevgeniy Brikman (O'Reily), Copyright 2019, 978-1-492-04690-5.) User Data is recommended unless you have more than 16 KB of scripts or if you want to deploy/leverage configuration management tools.