To me, make it count and earn it represent different bits of advice representing the same general sentiment.
Make it count, in my view, means 'do something meaningful with your life now that it has been saved.'
Earn it, similarly, means 'do something to pay for all the lives lost saving you.'
But they approach the idea -- ensuring the mission was not wasted -- in different ways that are not quite synonymous. In make it count, there is no sense that Ryan's life will start out in great debt, just that he should really cherish his life. In the second, the sense is that he is obliged to make it count, not for aesthetic reasons, but to repay a debt.