I'll probably get royally flamed, but I'm going to say 8-ball.
9-ball requires more difficult shot-making, but it requires almost no decision-making. "which ball are you shooting at first?...well, the 1-ball of course...and which ball are you playing shape for?...the 2-ball dummy!...and so on."
In 8-ball most of the shots are easier, but the decision-making (strategy) is much more difficult.
I also like how 8-ball gets harder as the game progresses, whereas 9-ball gets easier.
100% agree. 8 ball is a vastly more complex game than 9 ball. Here are the skill sets as I see it:
9 Ball:
--great shot making
--pattern style position...executing the same set of position routes over and over
--seeing the "angles" of position, and just getting close to that angle
--breaking with control
--identifying several basic safeties that recur over and over
--overall emphasizes the physical act of shooting the ball in with the correct spin
8 ball:
--far more decision making...ball selection is the difference between levels of 8 ball
--more *precision* cue ball placement required. you don't usually have "angles" and broad open position zones...you must play "point" position more often
--safety play is more complex. There are more balls to hide from. Also 8 ball emphasizes proximity safes more...where you play the cueball very close to your opponents ball to eliminate options. This is more dificult to execute properly
--deciding when to runout and when to wait...HUGE in 8 ball. This is why most APA players lose. They try to run out tables that are too tough.
--for the head ball break, typically requires a bigger break than 9 ball. For the second ball break, it is a very accurate and precise shot, I think a bit harder to execute well than the 9 ball break.
--overall, 8 ball focuses on the thinking aspect, decision making. It then requires quite a lot of finess and delicate perfect position play to execute.
To me, 8 ball is the more advanced game.
KMRUNOUT