I say both, but I'm most likely to say "No problem!" when I've done something for which I feel thanks are excessive, something that's just basic decency. To me, it feels comically magnanimous to say "You're so very welcome" or "My pleasure" for something as simple and expected as not letting the door close on the person behind me. However, I keep encountering "No problem!" as a pet peeve for some people, and I'm curious as to why that is. Is it just about preserving the traditional script of "Thank you" - "You're welcome"? Or is there something actually offensive about "No problem"?
Do you ever say "No problem" instead of "You're welcome"? Do you think this is a generational thing?