I think Bill hit it right on, maintenance is a key part. I typically run .050" just because it can be had anywhere around here easily, up to 24". After 24" I run .063". Only because the wider gauge is more readily available. I did have an old husky bar that I stepped up to .058 because the Grove was worn and would accept it, did the same with the .043 on my 193tc it runs. 050 now. Don't know about oz but our hardwoods don't cause any issues with whatever gauge you use. Oiling hasn't ever been an issue.
I've also heard, depending on chain brand, the drivers are ground down to fit the narrower gauge Grove and the rest of the chain is the same as .063. Never bothered to measure myself but can see this being the cheaper option for companies as the have less different parts on hand to make the different chains.