I chose the mages because it just makes more sense story wise.
In Val Royeux the templars basically behave like total d*cks and seem to have no interest at all to negotiate with the Inquisition.
Meanwhile Fiona (or at least what the Inquisitor thinks is Fiona) is civil and invites you to Redcliffe to discuss how to close the breach.
You don't know that any demons or evil metals are involved with the templars yet. All you know is that the Lord seeker is behaving different than he did in the past. For all we know at this point he could've just decided to be an total as*hole one day (which funnily enough is exactly what the real Lord Seeker Lucuis did after reading the seekers secret book).
So do you rather go to the mages, which seem interested in negotiations or to the templars which sit around somewhere, probably still behave like total jerks and could just easily say "Not interested in anything from you! Go away!" to the Inquisition again?
Of course, unless you are RPing as a total mage hater that doesn't want anything to do with mages, the mages are the smart solution.
And once you actually find out what is going on in Redcliffe "Crazy tevinter cultists on our doorstep that warp time and threaten to rip reality itself apart" are pretty much a higher priority than "We could still try to negotiate with the templars, who still sit around somewhere on the other end of Fereldan and probably still behave like total d*cks."