375 Ruger all the way! The trajectory is flat enough that making a 300 yard hit is no problem. I have a 2.5-8x Leupold on mine and with a handloaded 260gr Accubond at 2730fps I can sight dead on at 200yards and it's only 8" low at 300. With the scope on 8x, that makes the tip of the heavy part of the duplex my 300 yard zero. That's about as far as I plan to shoot this rifle, but it's capable of going farther.
Nothing wrong with a 338 Win. The cost of shooting the two would be almost the same though. You may as well go big!

Andrew