Well the 6.5/284 is a great long-range round, and actually will outperform the .300. It also kicks a bit less, or so I'm told. One potential problem, though, is that factory loaded ammo can be a bit hard to come by, whereas with the .300 you can get the Federal Gold Medal Match rounds. This will not be a problem if you reload. Personally, I'd go with the .300, but then again I don't reload.
Jake
Jake