I think this is more about  considering the consequences. You should build a decision tree here.

Regarding your thought process, I would expect something around the lines below.

Option 1: What does it mean to be impacted by a 50% smaller asteroid? Annihilation or survival? And what kind of survival? 

Being hit by a 50% smaller asteroid does not mean suffering a 50% smaller outcome. It may mean the exact same outcome (e.g. annihilation) or it may mean a much smaller outcome (some relevant damage but no threat to humankind or to life on earth).

If it makes no difference (between the 100% and the 50% asteroid), option 2 is better as you have a 50% chance of survival. 

If it makes a difference, then you would rather have a 100% chance of survival (even if you have to deal with a tough aftermath) vs. a 50% chance of total destruction.

(editiert)