Funny that this topic comes up because I very recently went through this choice for the first time with a Dalish elf. Every playthrough I did I chose the templars because:
1. Tactically it's a no-brainer risk-wise. I feel like such an idiot if I choose the mages.(...)
The rest is pretty much up to your personal tastes and involves some metagaming, but with this one part perhaps I can help since (in my opinion) going to look for Templars would seem like a really stupid move unless you metagame. I've seen some pro-templar path arguments that make sense (so my feelings became less radicla than they were) but the general opinion still is in favor of mages:
1. First of all, at that point of game you wish to close the Breach. The Templars supposedly have some power that can help you, but they're one-trick ponies. With mages you can improvise or try changing approach, or try another approach if the initial attempt fails. That's a point for the mages.
2. Mages actually came to you with alliance proposition while Templars are at best indifferent, at worst hostile. So considering them to be potential allies is really iffy. So the only natural thing to do is try and go to Redcliffe where Fiona doesn't remember you... and you learn about Venatori agent messing with some time-travel rift-related magic AND apparently trying to make the mages his pawns... Suddenly he becomes as big of a threat as the Rift itself since he obviously has something to do with it. You just can't leave him be if you want to stop the danger the Breach poses - this guy must be taken care of and quick, before he completely solidifies his position. He invites you - it's a trap, obviously, but also an opportunity you can take now or never have again.
Basically, you have a potentially more willing, generally more versatile ally OR an organization that doesn't want to have anything to do with you and while it supposedly could help you with Templar powers, they really provide you with that one option only. If they powers prove ineffective, they just can't help you, period - while with mages you can still work on some alternate solution. That would be enough for me to choose Mages with any Inquisitor that cares for his job first - even without the fact that turning your back on Redcliffe you pretty much leave behind an enemy that seems to have something to do with the very source of the problem AND takes residence in nigh-unconquerable castle with force of his own and recently "acquired" rebel mages. It seems that he'll only get stronger and you are given a chance to use his arrogance against him and take him out of the picture. How irresponsible is it to not take this chance?
For me at the very least siding with Templars, while defensible (after reading some pro-templar arguments here, at least) requires some mental gymnastics to justify, since we only really get to know about the corruption Templars are undergoing once we commit to their path.