Neither one. If you have to come on this forum and ask other hunters what scope you need for taking 300-500 yard shots, and you indicate you are considering what is more of a varmit scope in the power range you stated, you need not worry about what caliber to shoot.
I am not trying to bust your chops, but it is very obvious that you have not done enought practice shooting at extended ranges. You would have too great of a chance to wound a game animal at long range instead of being able to make a humane kill of that game animal.
Most hunters who practice enought to shoot at 300-500 yards will know that a good quality scope in the 3X9 or 4X12 power is more then sufficent to take big game at that range. Why short change you rifle's ability to take the fast short range shots due to choosing a scope that has too much power for most of the hunting you will be using it for.
Both of the calibers you mentioned will cleanly harvest deer at 500 yards if the shot placement is correct. It does appear that you do not know this, and coupled with your scope questions and the one you suggested, you have not done enought shooting at those long ranges and I suggest you reconsider until you can become more proficient shooting at those ranges and not do your practice on live game animals.
Like I said, I am not trying to bust your chops, or insult you, just trying to give you some sound advice on making you a responsible hunter towards the game animals you hunt.
Good luck on your hunt and sneak a little closer if possible.
RELH