Avis, I am a competitive long range marksman and have had several of the best military classes available in extended shooting skills. Both of the rounds you have listed are effective and accurate at 500 yards.For hunting of deer and elk in the states I use both. Both rifles have excellent ballistics and a strong following. I started out with a 300 on the 1000 yard ranges in the service. When I became a civilian again I began competing with a 7mag on the 500 and 1000 yard ranges. If your looking for hunting rifle I would say its a matter of personal choice and comfort. If your looking at target shooting from 500 yards and longer then you might look at other calibers that have been reworked and developed in the past 10 to 15 years. For hunting and constant 300 plus yard shots I would go with a 24 inch barrel. For a rifle that you may take out west once in a while a 22 inch will work as long as you know the rifles capabilities and your own. And finally if your going to want to become very consistent and proficient at long range shooting you will need to spend almost equal time at the range and the reloading bench. Customizing rounds for your rifle will take some time, and remember that humidity, elevation, wind, and heat will affect your bullet path. The 178 grain custom 284 diameter bullet I shoot in Texas for mule deer is the same I will use to compete with in Arizona or Florida. But the powder weight, seating depth, and even primer choice will change. Good luck and hope this gives you some ideas.
Retired Navy Armorer, master small arms instructor, rangemaster, 1000 yard competitive marksman, avid long range hunter, handloader, gunsmith, firearms restorer.