The actual comparison would have been, two of your sons each needed a kidney transplant or they would die, would you kill one person to save both of them.
It's a numbers game.
If the transporter had spat out three people each a combination of Tuvok and Neelix from the mix instead of a single super Tuvix, then by her rationality she would have let the Tuvix Corps live.
3 lives are more precious than two.
You know unless she was lying.
It's obvious that she just liked Tuvok and Neelix more.