It depends on who is loyal: if Tali is loyal and Legion isn't, send Tali in and vice versa. If both are loyal, send Legion or Tali, it doesn't really matter. If you send anyone else in, however, they'll die.
Great now Grunt died on the second part and he was loyal. This is retarded... I don't even care who dies anymore I'm not replaying anymore missions. they should make this a little bit more understandable..