Well I think it depends on the situation. If you're trying to post something they have already built good defenses against (e.g. child porn, or maybe mocking the king of Thailand), you'd probably have better luck with the handbills, but then you'd also have a better chance of getting arrested. If you are posting something that isn't automatically suppressed, I bet social media would still get you more reach.

You're missing the third option however. Start your own blog, then just post links on all of the major social medias, and especially on some of the minor ones. Once your link hits the right Signal groups or Truth Social followers or whatever, it will fly around way faster than any samizdat ever could.