Re: .300 WM or 7MM Rem mag
Shot placement is crucial and more energy will *never, ever* replace correct shot placement. Otherwise and all things being equal, more power!!!
OK, so right outta the gate, i admit i've never killed an elk - deer yes, elk no. However, i do have a 7mm RM and it's a nice cartridge - shoots flat and hits fairly hard (my longest shot to date on game with it is 520 +/- and it did the trick).
Additionally, deer go *maybe* 300-350lbs - maybe. An elk is, as i understand it, a different critter altogether. Maybe something that is more able to smash through heavy bone such as a .300 Win. is better medicine (bullet choice has ALOT to do with this BTW). To wit, the .308 caliber simply has more projectile choices, though both are easy to load for or find store-bought ammo if thats the plan.
I don't know how much more, if any, barrel life he would get with the .300 Win, but i believe it will be more than with a 7mm Rem (depending loads used of course).
Both cartridges can be very accurate - i wouldn't say one is more so than the other in that - depends on too many things...
Recoil is manageable in both, though the 7mm Rem has less - again, specific rifle has a lot to do with this.
Of the 2 suggested rounds, i'd choose the .300 Win.
Later on for 1,000 yds+ engagements on elk, the .338 caliber seems the way to go - it's what many here choose with great results.