On all of my SP's I have used Winchester brand 284 Win brass (Total of three 284 Win's, two of which are current [one center and one rear] and the other one from a long time ago
For a 1000 yard F-Class rifle that is currently being built for the world and Nationals at Raton, NM this August, I will use Lapua 6.5-284 brass and open it up.
The SAUM, WSM are also great options, with the SAUM's shorter length probably being an advantage. The negatives with the SAUM is finding brass and it is not as good as what you can find for the WSM.
My 7mm SAUM is an awesome rig, and I do lightly neck turn my brass. Reamer was purpose built with a tight tolerance neck.
I also have a 7WSM as well in a rear grip.
I would choose a cartridge based on what downrange performance you want or by what MV you want with said bullet.
As a minimum I want 1800 fps as a minimum impact velocity, and ideally 2000 fps.
Run the numbers with chosen bullet and from that determine what you would want your max hunting distance in ideal conditions, and then choose a cartridge/barrel length combo that will give that to you.
I don't worry so much about efficiency, but rather performance.
Well, being new to long range hand gunning I have only played with a few. The only one I have used on game is the .243 on a pronghorn hunt a few years ago. I have flirted with the .307/.308 in the Encore and the .308 in the Striker using both on steel at 550 (longest range available so far). My current steel shooter is a 6mm/.284 Encore. That said I'm getting ready to screw an XP together for long range deer/antelope and am looking very hard at the .284 Winchester myself, the other caliber I'm considering is the 7mm WSM. I guess my answer would be that I don't have a favorite yet.
I have a question for you though Ernie. Do you neck up 6.5-.284 brass for the .284 or use Winchester?