Saved Kaidan, killed Ashley. Boiled down to [IC] tactics for me, and by all accounts Kaidan was the more valuable of the two. Higher rank meant at the very least he'd had more experience, if it didn't flat out indicate he was a more qualified soldier. Biotics also had a part in the decision, as while anyone could potentially use a gun, Biotics had an added edge. And while I know this didn't play any real gameplay role, IC I saw a tower rescue as being more likely to succeed than going down into a pit where enemy forces were swarming, and keeping someone with the bomb to the end under those conditions acted as an added guarantee that nothing could happen to disarm it or quick-transport it away. (Again: I knew going in that those things didn't matter as far as the game was concerned, I just couldn't rationalize the character not taking those details into account). I didn't really have anything against Ashley, but when push came to shove I had to delegate her as a lower priority target.