I'd say a few things play into the decision:
1). Are you romancing Ashley or Kaiden? If you are, I'd save the one you're romancing. If you are romancing Liara or not romancing anyone at all, keep reading.
2). Is one of them important to your squad setup? Save the one that is more important. If they are both important, then save the person who's skills are least like your own. If this still doesn't fit, then...
3). Save the one you sent to work with the Salarians. The Salarians come back in ME3 if you save the person you send to fight alongside them.
Personally, I saved Ashley, but that was because she was my romance option in ME1 and stuck all the way through the whole trilogy. Then again, her character is more compelling, and she was a key squad member for me in ME1. Also, I always send her to work with the Salarians, and since I want their help in ME3, I save her and them. If I ever choose to play as a Soldier class then I may save Kaiden, but I don't see myself doing that anytime soon.
1). Are you romancing Ashley or Kaiden? If you are, I'd save the one you're romancing. If you are romancing Liara or not romancing anyone at all, keep reading.
2). Is one of them important to your squad setup? Save the one that is more important. If they are both important, then save the person who's skills are least like your own. If this still doesn't fit, then...
3). Save the one you sent to work with the Salarians. The Salarians come back in ME3 if you save the person you send to fight alongside them.
Personally, I saved Ashley, but that was because she was my romance option in ME1 and stuck all the way through the whole trilogy. Then again, her character is more compelling, and she was a key squad member for me in ME1. Also, I always send her to work with the Salarians, and since I want their help in ME3, I save her and them. If I ever choose to play as a Soldier class then I may save Kaiden, but I don't see myself doing that anytime soon.