Just by coincidence, I quoted that one yesterday in the Games thread about the Broncos-Saints game:
There is a difference between the two, though. Vitaï Lampada seems more straightforward, not to give in even in clearly bad circumstances; courage even in a last hope situation; a leadership role.
“Say not…” is more about discouragement even when others are still pushing forward with the struggle, and you don’t appreciate that your own efforts will contribute to the victory, even though that’s not apparent at the time.