I have used both, and IMHO, which one works best depends on the lock and the thickness of the flint.
One of my locks seems to work better with lead, but my other locks don't seem to care, not that I have done any serious testing.
To be honest, I don't know why this particular lock seems to spark better with lead, though it might only be my imagination.
In nearly 30 years of shooting flint guns, I have never seen a cock break, so IMHO, try 'em both and see if there is a difference.
Just kinda thinkn'...typn' out loud, so to speak...type.