For hunting? or long range target shooting? ie 300gr SMK?
Is the barrel shot out?
I would suspect the 30-378 is much more popular than the 338-378, not that ammo/brass would cost that much differently though.
I love the 338's, ie 340wby, for hunting, tough to beat the 340wby.
If for hunting though I would lean towards the 30-378 over the 338-378, for target shooting I could see some benefit to the 338-378