Out to 400 I think a 6x is about right for me on deer-sized game. Beyond that I can see the desire for a little more. As stated, an advantage to the higher magnification is a more precise aiming point. One advantage for a little lower magnification is better ability to see reaction at the shot as you don't lose them in the recoil as badly.
If you like variables a 3-9 or 3.5-10 should get you as far as you want to shoot.
I find keeping both eyes open helps a lot with seeing reaction to the shot. Loosing the sight picture due to recoil is going to happen with most big game hunting rifles without a brake or suppressor. Keeping both eyes open helps me not to loose track of the animal through recoil, and it's a lot easier on the eyes.
