Probably my child. I think that actually, the chances of my lover remaining my lover after I choose to let our child die is pretty slim. Maybe we can have another child, but would she doubt how strong my love is for that child when I just let our other child die? In fact, how would she then know for sure I don't value my life more than the both of them? My decision at that cliff will most likely make her question my priorities. The idea that a child's life is more valuable than the parents would certainly not be unique to her, it's drilled into most of our heads. I'd be betraying an indisputable concept to her (most likely - this really depends on our philosophy and relationship, but I'll just go with the "average" person).
Her friends and family may even help planting these ideas in her head, since I'm sure the decision would make them question me as well. She'll probably never feel secure with me alone with our next child again...
Don't forget this is midst the usual trauma that may ensue when a child is lost in a relationship and it appears to be somebody's fault.
Basically:
>It's pretty much guaranteed I'd have a child after I save him/her (he doesn't really get to choose to leave me lol)
>Chances are I won't even have a lover after I save her
And I'm sure I may be influenced by what the adults would want, I'd probably think "What would
they want me to do?" It sounds unfortunate, but we don't embrace a child's opinions in such matters because we think the child is too young to know better (which may be the case in a lot of cases).
Now for the mother, the reasoning is probably even more simple, if I choose her I'd be losing my "second family" entirely. My wife
and child would be gone... cruel it may sound, but prior to that event, I'd be spending most of my time with my wife and child and not my mother. They would have become a new part of my life, the future. There's many things my mother won't be able to relate to because of the generation gap, and parts of me and my life that I'll never be able to share with my mother. The bond between a mother and child is unique, sure, but so is a bond between father and child, husband and wife. The mother would probably feel horrible and always feel that she owes me something for taking away that part of my life. It'd be a terrible way to live the rest of her life.
Also, what would my second potential partner think if I let my child and wife die to save my mother

? I'm sure she'll respect the love there, but again, she probably wouldn't feel secure and comfortable around me. So it may even prove very difficult to get another wife, assuming I'm honest with her from the start.
Of course, all of this wouldn't have gone through my head if this did happen.