When it comes down to it, it's all personal preference. I really don't know that one is better over the other. The only big difference is leather tends to mold to the horse better but they all have their distinctive pros and cons.
I was told there was a recent olympic dressage rider that competed in a dressage saddle. I dont clearly remember the name but it wasn't a really well know rider. I thought it was quite neat.
The saddle is not what wins competitions. It's the riders and the horses. The tack just makes both horse and rider more comfortable and in touch with one another :)