I can agree with GG on that one. However, If you must have a big boomer, I prefer the 300 RUM at the 1K+ line. Out to 400 yards, it will make no differance whether it is 338 or 300. The fact of the matter is, the 338 case is slightly smaller. However, with its larger bore and increased burn area, bullet weight for bullet weight, the velocities are identical. Personaly, I would rather shoot the 200 ACCUBOND with a much higher BC at higher velocities than the 225 ACCUBOND. .590 vs .550 I have found these BC's to be close to the real world.
It is going to boil down to personal preferance, but at 400 yards, why beat yourself up and burn out those barrels??