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."
There's plenty of decision making in 9-ball.
All the decision making that is in 9 ball is shared with those of 8 ball. In addition 8 ball requires mapping your table where as 9 ball is pretty much color by number.
Much more strategy to 8 ball when played properly.
Frankly, I don't care for either game, but if forced to play one, I would be bored to death much quicker with 9 (or 10 ) ball than 8 ball.