You can't make that call and I would do everything in my power to save both. You can't pick and don't know the circumstances. If you put your mind to things you can become superhuman and achieve the impossible.
We all have an inner strength that most of us haven't tapped. I know if my family is in danger I would move heaven and earth to save everyone or die trying. In pressure situations one needs a clear mind and to think logically under pressure. I would go for the hardest first and collect the second on the way back.
You cannot choose one over the other and I would call it even. One doesn't warrant rescuing more over the other one. A mother may think differently and save her child first.