Re: 7mm Rem Mag VS 7mm Rem Ultra Mag
Whatever you pick make sure the rifle has a long enough magazine and a suitable twist rate for the bullets you want to shoot. Compare the case length, the cartridge SAAMI length, and the bullet seating depth you have to use to fit the magazine and throat.
The 7mm Rem Mag, 7mm Dakota, and 7mm Wby Mag all have fairly short cases, near 2.5 inches, To shoot heavy VLD bullets those can use a 3.6" magazine though it can still be tight. The 7mm STW and 7mm RUM have 2.85" cases and do not shoot 180 Berger well even in a 3.6" magazine. You can modify Rem 700 LA to get nearly 4" magazine space using Wyatts magazine mods. That takes a some gunsmithing but it's reasonable.
The required twist rate is a functon of velocity and air density. I shoot at a mile elevation in Arizona and twist is rarely a problem, but if you shoot near sea level in cold weather a 9" or longer twist may not work with heavy VLDs.
Check the terminal energy at the distance hope to shoot. Remember that energy is the square of velocity and drops rapidly with distance.