Terraform - should I use user_data or remote-exec?

Terraform - should I use user_data or provisioner to bootstrap a resource?

You should use user_data. The user data field is idiomatic because it's native to AWS, whereas the remote-exec provisioner is specific to Terraform, which is just one of many ways to call the AWS API.

Also, the user-data is viewable in the AWS console, and often an important part of using Auto Scaling Groups in AWS, where you want each EC2 Instance to execute the same config code when it launches. It's not possible to do that with Terraform's remote-exec provisioner