Deer are relatively small, lightly built animals that are not hard to kill, and anything from the .243 Win. up to .270 Win. is adequate.
In my book, a properly structured, 180gn bullet is the best all around weight in any .300 magnum. It will have better wind bucking capabilities and at long ranges will retain more velocity and energy.
But is it better for fallow than 150gn in the .300 WSM? Doubtful. When choosing a rifle, caliber and bullet weight combo, many things come into play.
The most important thing when taking any shot at game is the shooter’s degree of skill. I’ve cleanly taken sambar with a number of different calibers, including 6mm Ackley, .308 Win. .270 Win., but more recently with my 7mm Harvey Mag. and .325 WSM.
A stag hit in the left hind foot with a .375 or .458 is still just a wounded animal. If you can shoot your .300 WSM well enough, then you are well equipped to take any of our deer species.
A .300WSM loaded with a stout bullet is ll that is needed for all Australian deer species, including big sambar stags.