If you’re sure that you will never, ever want to have a web site with static data for your domain, I would use WWW for reasons described at Why use www? | www. is not deprecated.
If you ever want to set up Straightforward direct-delivery incoming mail, you’ll be very happy that you didn’t use the apex domain.
You can have your forum send from something other than the forum domain name, but you’ll need to edit the app.yml by hand and/or find some other way to change the notification_email address.