Mix of both. The earliest SR9c's had a different striker spring length than they do now, and that old length could contribute to light strikes. However, the ammo that was always present in the SR9c light strike issue threads was the Winchester White Box 115gr FMJ, or simply WWB. Coincidence? Culprit?
You have two options, A), call Ruger and see about getting a new striker assembly because you're getting light strikes. They should just ship you one no charge, but they might ask that you send it in (ask about a prepaid label if they don't mention that). Or B) just stay away from WWB ammo and find another suitable range ammo. I prefer CCI Blazer myself (aluminum) and others like Federal stuff. If all you can get is WWB, try the updated striker assembly.
You could just do both. My 1st Gen SR9 has a lighter striker spring and doesn't fail with WWB. I'm among the small crowd that believes the problem was the ammo. Others disagree and say Ruger is to blame. It should feed and fire everything though like all other Rugers.
Josh