Which ones are "better" you ask. That's simple. GAMBLING.
If you value the higher average level of play then you would be wrong, because the higher average level of play comes from the best tournament players. If you value stamina above the highest level of average play then you would be right, it would be the long set gamblers. Again, this is really no different than the question of "who is faster" or "who is the best runner" between Usain Bolt (best sprinter in the world) or Geoffrey Mutai (arguably best marathon runner in the world).
If you most value the absolute highest possible top speed, then you would think it is Bolt that is the fastest in the world, because he wins in the highest gear/able to cover the most meters per second. If you most value the ability to keep up a fairly high average speed over a long period then you would see Mutai as the fastest or best, because he is faster than Bolt under those conditions where stamina becomes a factor instead of just your sheer top speed.
I don't see how you can say there's less pressure gambling, when most guys are firing their whole bankroll, opposed to a tournament where there is a set buy in.
We are talking about pros here. Firing their entire bank roll is the exception, not the norm. In fact it isn't even the norm that pros gamble against each other with their own money. They are usually backed when gambling against each other, which takes away tons of pressure.
Even in the cases where they do bet their own, they always know that they can continue to flip it for another set. Or in the case of a long set, they know that there are a ton more games to be played and they don't have to worry nearly as much about any particular mistake.
And I think you are thinking about this in terms of a single race to nine compared to a single race to one hundred. You are forgetting that the best tournament players have played thousands of races to nine or eleven in tournaments. One race to nine doesn't necessarily tell you much, but many hundreds or thousands do. And they generally happened under more pressure too, which makes it even more telling.
Another example, lets take John and Jack. They play eleven races to nine against each other in various tournaments over time. One race to nine may not tell you much about how they match up or who is better, but after eleven of them you get a pretty good feel for who is better. Certainly as good of a feel as say one race to a hundred anyway, because the total number of games played is going to be almost identical. There is one big difference though. In the eleven races to nine every game was more important, every mistake more costly. There was a ton more pressure. There will never be any periods of time where your opponent goes into give up mode, and you will never have periods of time where you get to coast with no pressure. It is all pressure start to finish and better be your best performance from the first ball to the last because any one mistake could cost the loss of that race.
Now a race to one hundred does do a better job at determining stamina, but I don't care who has the most stamina. If that was important they should just run a marathon against each other and forget playing pool. I want to see who has the highest gear, the highest levels of pool playing, under the most pressure, and you get that much better with many shorter sets than a single long one. I think it is easy to argue that they guy that is winning a ton of shorter sets (like a tournament player who is dominating tournaments) is a better pool player (maybe not better stamina, but a better pool player) than the guy who is dominating long races gambling, because he has to do it on demand, under more pressure, where every mistake is more costly, and where he knows that he won't be able to flip the coin again for a second chance.
Many small sets is always a better determiner of skill than a single long one. A match in a tournament comes with more pressure than a similar length set done while gambling (unless you are betting a very large amount of your own money).
