Access to a 1500m range?
Do you have access to a range greater than 1000m? If not, then the .308 is less expensive, and probably better training. A .338 would make 1000m shots much easier; a .308 would take more skill.
You could always downgrade to a 6.5 and get similar ballistics to a .300 wm, and a lot less expensive ammo for practicing.