To be honest its not even close. 8 ball is a much easier game to win giving up games. Especially when your playing someone who isn't going to run more than 4 balls very often. There are people who consider themselves "9 ball players" and even if they don't realize it, their winning percentage would be higher in 8 ball.
Apa changes 9 ball to something else, so you can't just use their numbers.
The best evidence of this would be the NAPA league. They play 8 ball, 9 ball, and 10 ball. (Not in every area)
The skill ratings are determined by winning, and you get a rating in each game. The handicap formula determines skill rating the same way reguardless of which game you play.
Almost every "good player" has an 8 ball rating that is higher than their other ratings. Also 10 ball is slightly higher than 9 ball, but that's a different conversation.
The only time this is not true is when players don't have a proper amount of matches played.
An example would be NAPA's highest rated player Jason Kirkwood. His 8 ball handicap is in the mid 140's, and his 9 ball is in the low 120's.
9 ball gives the weaker player more opportunities to win. It's just the design of the game. You can't really beat yourself playing 8 ball vs. weak players, you easily can in 9 ball.
Look at it this way, an Apa 6 is roughly a 70 in Napa. Playing Jason Kirkwood that's a 10-2 race. If you were betting money on the lower player would you rather the player pick 8 ball, or 9 ball? It's an easy choice right?
By the way, for those who don't know Napa has all game stats online. You can easily look the matches up.