Re: 300 RUM to 338 Lapua Mag?
The 338 Lapua, 338 RUM and 338 Edge all have similar ballistics/velocities within 100 fps, or so. They all use slow burning powders such as H1000, Retumbo, etc. So barrel life will be about the same with all of them. How hot you load the round will have more bearing on barrel life than any of the three calibers.
What is true, is that you will get twice to three time the case life using Lapua brass. When you factor that into the cost of the brass, plus the inherent quality of the brass (read less case prep time), and the ability for Lapua brass to withstand higher pressures - the 338 Lapua is at least on par with the other two above.
Anymore, the calibers I shoot are determined in large part by whether or not Lapua makes brass for it.