Re: 338 Lapua Mag or 338 RUM
6 of one, and half dozen of the other really. They use pretty much the same amount of powder. Both can be made to be very accurate. So I would say your safe either direction. I shoot a Lapua occasionally and feel due to the brass quality, you can get a little edge toward velocity verses the RUM. But head to head you really shouldn't see much difference.
"Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it.  But small is the gate and narrow the road that leads to life, and only a few find it.
If you find your self in a fair fight, your tactics suck!- Marine 1st Sergeant Jim Ryfinger
Friends don't let friends develop canonitis!
arguing over the internet is like the special Olympics....even if you win, you are still...special!