I have both. I wouldn't trade my 6.5-284 for anything. It is just plain scary accurate but I reload.
Build yourself a switchbarrel rifle of whatever brand you wish. I build on LA Savages or Remingtons. Pick your components and build exactly what you want.
Both are easily accurate past 1200Y
300:
Massive amounts of commercially available ammunition if you don't reload
Ammo will always be available over the counter.
Probably cheaper per round to shoot because of this. (Can not confirm because I haven't purchased over the counter ever.)
Easily accurate as far out as you want to shoot but belted magnums will always be a bit less accurate unless you reload and headspace on the case neck instead of the belt.
A bit more recoil, suppresses well
Hunting, will easily take anything in North America
Savage LA, Rifle Accutrigger, Accustock, X Caliber 300WM stainless heavy sporter barrel 1-10 twist 5R, Northland Shooters barrel nut and recoil lug.
6.5-284
Ammo selection over the counter is limited. Availability is probably even more limited.
Also easily accurate for any distance you want to shoot
If you don't reload you won't be able to take advantage of the Norma chambers length for seriously long bullets. My OAL is as long or longer than my 300
If you do reload, 4831SC will be your friend. 140G Bergers are amazing at distance on targets, on animals, etc. I hunt with mine exclusively now.
Less recoil, suppresses nicely
Higher cost per round I think commercially. Reference above.
Savage LA action, Northland Shooters barrel nut and recoil lug
Rifle Basix trigger
MDT + AI 300WM magazines work perfect for the 6.5-284
X Caliber carbon fiber 1-8 twist 5R