Fiona was being manipulated by Time Magic and misinformation. Others were not.
This. People either fail to understand or refuse to understand that when someone messes with time and performs time travel, it usually results in alternate timelines where the same people are different.
As I explained previously, Leliana in the Dark Future is not the Leliana we know because we are in a different timeline thanks to Alexius. Similarly, the companions trapped in the dungeons are not the companions we know because they are in a different timeline, a different reality. The Fiona we see in Val Royeaux, the Fiona we meet in the Redcliffe Castle and the Fiona we meet in the Dark Future are three different people from three different timelines, courtesy of time magic by Alexius.
For reference, think of J.J. Abrams' Star Trek movies. In the first one, Spock says that Nero's meddling with the timeline has created a completely different / alternate timeline where the fate of everyone in it have been altered, making them different people. Heck, we get to meet two very different Spock in the movie.
The Rebel Mages were manipulated by one of the, if not the most fundamental aspect of reality - Time. By contrast, the Templars were manipulated by the one of the many things they are supposed to fight - Demons. In addition, the Templars were manipulated by their order's framework - That is, they follow order from superiors without question.
So let's weigh those two:- Reality Altering Time Magic vs. Demons & Inherent Flaw Within Command Structure.
Couple this with the fact that Redcliffe is close to Haven, that we have (or rather Leliana has) an intimate knowledge of the castle and with the fact that it is under the control of Tevinter Magister. On the other hand, we have Templars in Therinfal Redoubt, a secluded training location far from Haven / far from civilization for that matter, which quite frankly looks like a form of training exercise of sorts and the requirement to meet them is far more demanding (we have to recruit nobles)
There is no contest - Mage path is superior.