Any of them can be depending on the opponent. 9 ball race to a 100 against prime Earl Strickland is in another universe of difficulty vs. snooker against someone who has never played. Now if my snooker opponent was Ronnie, I have an equal chance of defeating both of them. 0%.
That's the point I'm trying to make. Difficulty is mostly defined by opponent (and rule set. We can see how the difficulty can change if we make a 9 ball competition best of 5 games, where a "game" now means race to 7). If you're asking which of the games presents the harder solitary challenge in making balls, I don't think we can really answer that because comparing a 50 point run in snooker to running a rack of 8 ball isn't 1 to 1. It's harder to make balls in snooker, but pool requires that you make more of them for a similar level of achievement.
Intuitively, I would say there's a similar amount of people in the world who can go 150 and out in professional straight pool competition as there are who can achieve a 147 in professional snooker competition. The poster Poolmanis here is a good litmus. Last I remember, his high run in straight is 119, while he's achieved (a ball in hand) 147 and 132 runs in snooker. So for a talented cueist, centuries in snooker and 100 ball runs in straight might be of equal difficulty.
Opponents are irrelevant when comparing the difficulty of a game.