By harder what do you mean? Winning at or playing?
Anyone can pick up a cue and shoot ball, or roll a ball down the lane. Mastering it is completely different.
Pool is a game that depends on your opponent. The opponent has more affect on the out come than bowling.
In bowling everyone has a set amount of frames. It doesn't matter what you do you will still get another turn and what your opponent does will not affect your next turn.
In pool there is no guarantee that you will ever have a turn.
To be great at either takes a lot of practice, and focus.
Either game has a lot of bench marks. Bowling has it's perfect game and pool has it B and R.
You see a lot of 300 games but not too many 900 series. You see a ton of break and runs but not too many 150 runs or perfect snooker scores.
I would say pool overall because of straight pool and snooker.