I am an avid student of history, and have read many, many works on WWII; and being in my 60s I grew up with people who fought in that war, and I’ve had myriad discussions with them over the years.
The one clear and overwhelming conclusion, from almost every source, was that the US Marines were great fighters, and truly gung-ho; but that the Aussie light infantry — especially in New Guinea — were without equal.
I had read somewhere that Aussies were considered “half a soldier” before even joining the army; the rough life in the outback really toughened them up.