Tag Manager is definitely preferable in my experience if you want to empower non technical people such as marketing to make their own changes on the fly without having to bother developers.
Yup, GTM is great until the folks in marketing add a script to the site without first testing in preprod that causes the UI for the app to not render.
You clearly have a more skilled marketing team than the one I tried to work with using GTM. I ended up dropping it because rather than implementing tracking Javascript in a text editor I was having to do it in an obtuse GUI instead - marketing wouldn't go near it.
It's not so safe, if not developers just copy paste code given by a third party they might inject insecure JS code