There are people in this discussion posting opposite experiences from you, so it's probably all anecdata either way. But I think it's a convincing argument tha

There are people in this discussion posting opposite experiences from you, so it's probably all anecdata either way. But I think it's a convincing argument that SMS is significantly more vulnerable to spam because the price of entry is negligible and it lacks the traceability that comes with a requirement to have an Apple ID to use iMessage. I hope you're reporting the spam you get to Apple, because the fastest way to stop it is to have the offending account terminated.