With the mage you get a couple extra dialogues (I might miss some)
- Perseverance
- First Kiss
- Lake date
- Abomination talk
With the Templar you get a lot of extra dialogue just going with the class that isn't romance-related, but if you are in a romance, it takes on a whole new level (moreso if he is also taking lyrium):
- When Cullen first tells the IQ that he stopped taking lyrium
- Perseverance
- Prayer Scene
- Pre-Spec talk
- Post-Spec talk
- Post main campaign talk (haven't gotten there yet, but have heard about it)
I'm not sure if there are any changes during Before the Dawn. I will have to let you know on that.
I understand how some like the mage dynamic, I just don't. Most of the extra content is the IQ questioning him over and over..."I'm a mage, is that ok???" "You're alone with a mage, is that ok???" When I played through as a mage it actually started to wear on me like a girl constantly asking her b/f if he still thinks she's pretty. With a Templar, yes there's more angst, but to me it feels like they're challenging each other as equals, rather than the mage always needing confirmation of his feelings. I guess it's just a self-confidence thing? I dunno, that's just how I see it. Folks are free to play the game however they wish. That's just my take on it! 
*is Pro-Mage and conscripted them rather than offering them a full alliance* Lol, I am a stout supporter of the mages, but I wasn't stupid enough to offer them a full-alliance. I guess you can say that I believe in the fair treatment of mages and the freedom of mages, but I also felt that Fiona and her people needed some kind of punishment for joining Tevinter. Which led to many innocents being homeless, killed, or go missing.
As for my opinion on the whole CullenxMage thing, well, lemme start out with how I feel about playing mages in general. Mages get more dialogue and lore all throughout the Dragon Age series and we all can debate this until we are blue in the face, but mages have more lore/plot significance in the presented games. DA2 was meant for a mage if you wanted full immersion, DAI had tons of magical aspects and lore with the unnatural, and DAO was the starting point for all of it.
So I feel that mages often get far more plot/lore when compared to rouges and warriors.
I like CullenxMage because that is my own personal bias. I go weak for some of the tragic or ironic romances and that is what makes me fall in love with Cullen and a mage. I think it all depended on how you roleplayed your character and who your character was, in the terms of how whiny or how grand the dialogue was. My mage, Mira*who you can see in my story* is not someone who is going to be whiny and concerned about what others think about it her; at least not in her present state. She still has moments of doubt and Cullen has doubts about mages as well, so I felt some of those dialogue options were perfect for her.
In short: I feel as if everyone has a different experience based on who their character is and how that character feels about their own magical gifts. It's not about which romance is better or what(Templar or Mage) Cullen is meant to be with, but who he is meant to be with. I think Cullen is meant to be with a strong, patient, understanding, and at times emotional woman. One's character can be either a Mage or a Templar and still have a great romance with Cullen.