Have you read the other replies? You’ve posed a fairly binary question, when the answer is really, “It depends.” The answers depend on the weapon, placement on the body, and user preference, not to mention the cut of leather you’re using. 9 oz belly will never work as well for a holster as 6 oz shoulder, for instance.
By way of example, I usually carry a CZ P-01 Omega with a dot and a spare mag, all AIWB (i.e., up front inside my pants). It’s a weighty rig, for the overall dimensions of the firearm. 7/8 works just fine for it. Wouldn’t use less than 6/7. I’m working on an updated design using 8/9 shoulder that will be really tough and will take carving nicely. Do I need it that thick? Not really. But it will be bombproof, which is what I wanted.