To not make Claire kill him you have to always show her that revenge is not the way to go, since the first mission that she mentions it else she'll just shoot him regardless and you can't get it, if she spares him you get both beast and sampson's car.
If she kills him, you get beast.
If you continue the race you get neither.
Afaik you -need- to have Claire confront Sampson but not let her kill him, else you don't get any of the two cars. (This is what I've read off a guide anyway, I just had her kill him lols)