I would build the one your likely to reload/shoot/practice with more often. If it's a 100 round a year gun, unless you have .05% percentile shooting abilities, you won't develop or posses the proficiency to use either chambering to it's full potential, let alone see the energy/wind drift advantage of the 338s over a 300. Hope that doesn't sound preachy. I've learned though my own experiences proficiency with THAT rifle at distance only comes through practice. If lugging a 18# rifle out on the weekend, or throwing 100 grain charges, or beating yourself into the ground with recoil becomes a chore, you likely won't shoot it enough to develop any proficiency with it. So which one do you think you'll shoot more? That's the one I'd build.