Your original question asked which one you should care as a "primary go-to platform" if you could only carry one in an emergency, and you limited the choices to the AK and the AR. You then gave the application of having to shoot at 500 yards.
Answering only your question(s) here, I'd choose the AR, preferably something in 20" and with 75 or 77gr ammo, as others have said, to get the terminal ballistics.
Expanding a bit, if I could have the freedom to carry TWO long arms in an emergency/survival situation, I'd probably want an lighter M4/mid-length AR, plus a lightweight .308 bolt rifle such as a "Weather Warrior" Savage 16, which only weighs about 7 pounds. Then I could use the tool most effective for the job at hand, and with those two rifles, you could do everything from nailing a rabbit for supper to shooting through walls. The .308 rifle would be the ideal choice for the 500-yard scenario.