Kaidan because Ashley is useless once you get James.

Did Ashley on my first few ME runs years ago and I remember her being hella annoying and always messing up so I saved Kaidan on a whim one time playing fem shep and that boi has some depth, decent writing and much less annoying than Ash ... So Kaidan survives every ME playthrough I do now.

You know it's still the singular hardest decision I make in ME1

Ashley because Kidan is a wooden post. She at least has some familial depth that comes to a plot in 3. As far as useless, she is far from it just have to know how to use her.

I like them both, to begin with. But I'm gonna prioritize the survival of the person managing the nuke and Ash makes a really good argument for why the techie should go with the tech and the soldier should go with the soldiers. So I wind up saving Kaiden and my Colonist Shepard gets left to grieve her almost sister.