The one thing GTM has over DTM is that there are a bunch of hosted pixels by companies like Facebook. Otherwise, it's incredibly inferior. The DTM UI is easier, more intuitive, and more comprehensible. Debugging is easier as well, and the triggers are better defined in DTM. Version control is far superior and it's so easy to tell what tags are firing and when. I went from DTM to GTM when switching jobs and was so relieved when my new company made the switch to DTM. I'm much happier and it's much easier now.