It's a very different debate depending on whether you're talking about the difficulty for the elite professionals or your average run of the mill player.
Luke Littler threw a 9 darter 4 times in competitions in 2024 and probably 10+ times in practice.
Ronnie O'Sullivan has the record of 15 maximum breaks in competitive games but that's over a 30 year career so an average of 1 every 2 seasons. Again he's probably had many, many more in practice.
So at an elite level the 147 seems to be statistically harder and if it's that hard for someone like Ronnie, imagine how hard it is for your average person that plays for fun.
So the only answer to the question is the 147 is harder.
........ Now what about a hole in one?