Moodle releases are generally very stable. For me, the paramount consideration when deciding whether to use LTS or non-LTS branches is what plugins I will be using on the site, as third party plugins may not be compatible with later Moodle releases, in which case having the site running a LTS version may be preferable. 3.9 is currently receiving all bug fixes, so it's fine to run it, but when it hits security-only, bear in mind that non-security related bugs won't be fixed.
If you are running Moodle core with no third-party plugins, and use Git to manage the site, I see no reason why you would not stay on the latest stable branch. The only other exception may be for *very* large installs.
If you are running Moodle core with no third-party plugins, and use Git to manage the site, I see no reason why you would not stay on the latest stable branch. The only other exception may be for *very* large installs.