biggest difference I have found between the 7mm mag and the 300 rum is the recoil. the 7mm is a snappy roud, no doubt about it, especially when using a lightweight slim contour hunting rig; but the 300 rum is downright punishing to shoot for extended periods unless you are accustomed to shooting very heavy recoiling rifles like the big magnums or the afican game loads. my 7mm is a lightweight stainless/synthetic rem 700 with a 22" slim barrel and a super lightweight stock. It heats up after a few rounds so rapid fire benchrest shooter it ain't, but recoil is acceptable with no muzzle brake or special recoil pad. The only 300 rum I shot was a similar rem 700 with a standard (sporter) weight barrel and a wood stock and it was uncomfortable to shoot after 10 rounds; to the point that it wasn't fun shooting it any more.
Unless you are comfortable with shooting heavy recoiling rifles, choose the 7mm mag.