I don't see 9 ball being the same type of challenge as snooker, where shot making is REALLY put to the test on a much bigger table with much tighter pockets, and where it requires much more precise position play too. If you really want the ultimate challenge, they need to play snooker. Of course this wouldn't be fair because of Quentin's prevous experience and expertise in that game. The same way that 9 ball wouldn't be fair because of John's previous experience and expertise.
No game has all of the possible pool skills or shots or strategies, and somebody can make a valid argument against any particular game not featuring certain things that they feel are important. That is why the above argument for 9 ball is full of holes. It does not showcase the best pool skills, just the ones that you happen to want to see, or happen to value the most.
For anyone that actually cares about seeing a fair game, as opposed to the game they just want to see or the one that favors their favorite player, then 9 ball, snooker, and most other games have to be completely out of consideration. Since no game features all of the possible shots, strategy, and knowledge requirements anyway, it makes even more sense to just focus on finding a game that is fair.
The only game that does not give a huge advantage to one player over the other (except for a small advantage due to the equipment) would be American 8 ball, which Quentin offered to travel around the world to play on American tables, with American cues, in John's backyard.
Or even better than that, they could just play a fair all around match with three of John's games of 8, 9 and 10 ball on American equipment, and two of Quentins games of snooker and English 8 ball on their repective equipment (slightly in John's favor at 3 to 2). This would have the most appeal, to the most people, and it also features almost all of the pool skills that are possible. Quentin has offered this game too.
So what's so wrong with playing the one game that is the most fair, even if it doesn't happen to be the game that you would like to see?
And whats wrong with the all around that Quentin offered? It would showcase almost every pool skill imaginable, has tons of appeal to the masses, and it does it while being fair to both participants.