I've spoken to a couple of people who use R44's for mustering, their comments are that it's significantly more versatile.
The R44 has much more margin, so a significantly reduced pucker factor, particularly 2 up and the cattle tend to respond to the different sound quite well.
It's also much better for vermin control, as you can put 2 shooters in it, will lift lots more hay bales when it's flooding and carries a couple of extra chicks when the B & S is on.
Cost is only a major factor when the cattle prices are really low, so you don't use it for stragglers.
Having said all that, a cleaned up 300 with a tubo diesel and a good air filter.... Anybody listening out there????