No offence to Gaspard supporters, but I am going to pop in here to question why anyone would support a warlord over a cultural world-leader xD
If it's strictly for elves, I'd honestly go with Briala/Gaspard. Briala/Gaspard is basically just "Briala", with Gaspard as a figurehead, instead of her simply at the sidelines. Rights are given to elves almost immediately.
Isn't Gaspard prideful? He openly revolted against Celene. How long would it take for him to get fed up with being controlled by an elf? It certainly doesn't create peace in the empire as it remains controlled by deception/blackmail/threats...
With Briala/Celene, elves are given rights immediately as well. Briala herself is invited to address the court, and is made a marquis by Celene and given land. Not only that, but they continue to improve the lives of elves together, openly.
Gaspard may be a better puppet (for a time) but Celene and Briala can become actual partners and that leads to true equality and acceptance for elves.
"Celene will never do as she is told, your advisors even tell you this. Gaspard can be completely controlled by Briala. If you choose Gaspard/Briala, Briala and the elves show up to fight with you in the Arbor Wilds."
Of course Celene will never do as she is told: She is a proper Empress, she doesn't take commands, she gives them. She can be worked with using diplomacy, but she can't be bullied into becoming a puppet. Not to mention... Why did Briala send her people to the Arbor Wilds? Orlais has an army she supposedly controls: Did she really send her own people to fight? I suppose its the gesture that counts, but I thought she was out for protecting her people, not sacrificing them.