As always, it pretty much comes down to personal preference. As long as it fits the horse properly, they
don't care whether or not it's leather.
I have both a leather Stubben and a synthetic Wintec. Love 'em both. The Stubben is JJ's saddle, and the Wintec is Casper's. As Val stated, there isn't a whole lot of difference in the weight between leather and synthetic English saddles.
A badly fitting, high dollar leather saddle is much worse on the horse than a well fitting synthetic one.
If that's what you prefer and it fits the horse, all the leather snoots can go pound sand.