Looking at your choices the 7mm Rem Mag with 150-160 grain class bullets at 3,000+ fps will be plenty fine for dispatching the plains game you defined. Would bet most shots will be at or inside 250 yards. Setting a 250 yard zero is very useful for the 7mm Rem Mag thereby eliminating any real concern for holdover or hold under for most shots encountered. No need for the 300 RUM unless you want to absorb more recoil than necessary with no real benefit. Best of luck.
