Templars, I sided with the mages in my first playthrough, but I knew that I should have went Templar.
The Templars were forced to become Red Templars by drinking or eating Red Lyrium.
The Mages were not being forced to serve Tevinter or Alexius. Sure, you can say that they made a deal or were indentured, but in the end, they still had their minds and they decided to fight for Corypheus.
Also, if you talk with a lot of Mages, they say that not even half of the all the Mages wanted to rebel, many would prefer being back in the Circle.
If you played DA II I can see why you'd side with the Mages, but Meredith was an exception and that was only in Kirkwall.
Morally, itd be best to side with the Templars.
Gameplay wise, who cares, just play the game again and make completely different choices next time, we all end up doing that anyways 