There is on average 200 fps difference between the 338-300 RUM and the 338-378 wby with 300 grain bullets and equal pressure. For every 338-300 RUM that gets 2900 fps there is an equal number of 338-378's that get 3100 fps. Most averages are 2800 and 3000 respectively. You are comparing apples and oranges with two completely different rifles designed for different purposes. Both are excellent cartridges for what they are designed for. The wby is a big boomer with noticably more power and cost. You must decide if that is worth it for your purpose.
Today I was testing my rem 700 338 RUM at 3300 fps with a 225 grain .64 bc bullet. That rifle is about $650 and will kill anything you can hit with it. It maintains .5 moa accuracy. I would recomend the 338 RUM over the 338-300 RUM since they have the same performance but the factory round has several advantages.
The 30-378 is a good deal and you could later build whatever you wanted off that action. Excalibur improved, Lapua improved, 338-378 and imp version. If the 30-378 shoots you are good where you are. Then rebarrel when you shoot the tube out. It would be a great platform.
I own most 338's talked about on this forum and shoot them regularly. Nearly all of my hunting is done with three. The 338-378 wby, 338 RUM and 338 winchester. The 338 winchester is a light backpacker that does not require a brake. Longest kill 1104 yards on a moose.
The 338 RUM requires a brake but the 700 remington is still a light carry rifle that goes hunting instead of the 338-378 unless I am in an area where I need a serious long range rifle. Longest shot close to 800 yards on an elk but it will easily kill as far as I can hit something.
The 338-378 wby is my favorite long range hunting rifle. 127 grains of H-870 will drive a 225 grain .64 bc bullet 3500 fps out of my 28" barrel and long range hunting is way fun with it. It weighs 10.75 pounds which is heavy for me to backpack with although I do when I must. Usually I carry it when I am hunting near a road or a pack in camp, fly in Alaska/horsepack/ATV/4wd/etc. in other words it is my preffered weapon if I can. Longest shots are several 1100-1200 yards.
So, the reason I said all that is that you understand you can take a 338 winchester and make as long a shot as most will ever shoot. The RUM's just do it a little better and the 338-378 wby does it a lot better. Less drop and wind making long range hits easier. That is what you are paying for. You just got to decide how much fun you can pay for to make it easier on you with long range kills.
)
