A decision had to be made. I'm sorry.

I wasn't even sorry to let Ashley die. She was condescending, hostile, and antagonistic for no real reason. And she's supposed to be a love interest? If you're playing anything but a full renegade, it's hard to imagine she'd even like Shepard.

Kaidan's characterisation fits better regardless of your Shepard's alignment, he's a more useful squadmate in general (especially in ME3), and he's better voice acted.