I almost always save Ashley, but not because I don't like Kaiden. To the contrary, I think he's a great guy. The thing about those two is that when you meet them in the first game, Kaiden is level-headed, well-adjusted, and fairly intelligent. Or, as so many others see him, 'boring.' I'll let you guess if I agree. Ashley, on the other hand... is not. She is not in a good place by the first game. At all. Kaiden has, by and large, sorted his shit out, and is at peace with his life and the galaxy in general. Ashley still has a lot in her life to take care of, so I give her the chance to do so.
Basically, I feel that between the two of them, Kaiden has the least regrets in his life. Ash has many, and needs time to work through them. So I help her do so.