338 RPM head stamped brass is easier to find than 338-06 head stamped brass, (although I have some). I also have 30-06 necked up and 35 Whelan necked down brass. Why? just more speed and power, plus something new and fun. Yes, there is a case capacity difference with the RPM on par with the 338 Win.
338-06 brass is cheap thanks to 30-06. Why would you even want to go to more difficult to find brass thats exotic and spendy? Is there any real case capacity difference between 338RPM and 338-06?
I've got 338win and 338RCM and really like both but currently there's no supply of brass other that I've stocked up on. Seems 338-06 would be the choice for economy and logistics to keep it running.