Hmmmm... I found this to be a pretty difficult choice, I ended up voting for Gaspard, but I wouldn't say that I am a massive Gaspard supporter, it really came down to how he was very consistent throughout the book on what he was doing and what his goals were, where as Celene seemed to change her goals, based on Briala. I don't think someone with that much power should be basing choices for the entire empire based on the what their lover believes to be the right.
If Celene was helping the elves for completely selfless reasons, and not based on her love for Briala, then I might have picked her. I chose the elves for who I wanted least, purely because I don't think they are in the running for control of the Empire. If the elves were in the running, I would pick Briala over Celene, not sure if I would pick her over Gaspard.
Of course my opinion might change depending on any info released on the matter, or when I get in game and am actually faced with the choices, but for the moment that is what I think on the matter.