Let's say you're in a situation where only one person is coming out alive and you know it. Would you save yourself or the stranger? Why?
Edit: Think of it as a split-second decision. For example you somehow tripped over the stranger and you both fall onto the tracks of the subway and have time to either climb out by yourself or help the stranger up and die.
If you look at the lore.
The stranger is most certainly the Exo that the FWC either sends into the future or gives visions of the future to.
The stranger is not a guardian but has the technological advantage to survive in Darkness areas.
The stranger is in contact with someone else of knowledge and power. Likely Osiris or a time shattered Kabr.
Despite all this, in all of the scenes strangers interactions are shown to develop your character. Make sure you have the knowledge and skill to keep going.
This means that you are more certainly of higher value to the future of the man/exo/awoken.
So I'd save me.