the top end on the 30-378 is about 3800 fps for a 180 grain bullet vs 3550 for the RUM thats the diff.
I personally perfer the weatherby but to each his own.
3800 FPS?

?? I don't think so.
I own a 30-378 weatherby and absolutely love the gun. I also just purchased a 300 RUM for my brother and I am going to tinker with it on reloading.
The factory ammo is extremely high priced. If you don't reload. Purchase a RUM. Still, factory prices are high but not as high as the Weatherby.
Brass prices are not that bad for this gun. I was getting 8-10 reloads ouf my brass before I saw signs of separation. That brings the cost down considerably. Remember you are buying Norma brass vs Remington brass. You are stepping up in quality.
Get the Mark V action if you get the WB, it comes with an excellent factory brake as well.