I've killed animals up to and including elk with a .300 Wby Mag, a 7mm Wby Mag and a .270 Wby Mag. All died fast and no tracking was needed. None, as far as I can tell, knew the difference in what hit them. In fact, all the longest shots were made with the .270 version, including a devastating kill on a 5pt bull elk at 275 yards.

In my experience, .300 mags hurt when I shoot them, and most 7mms don't (except for the RUM and such). Between your choices, I'd go with a 7mm mag just because it has noticeably less recoil than .300 mags of comparable case size. You will shoot more accurately, and that will make a big difference in performance.

But, like Doe Dumper says, you really aren't going to see a great deal of increased performance from either one over your trusty .30-'06.