One of the reasons I stand up so much on high magnification scopes is that I do more coyote hunting than anything else. Coyote hunting at mid to long range, for me at least, is a very different thing than big game hunting at that range - visually - so I have different expectations and requirements from my optics. While pronghorn are larger than coyotes, they're still considerably smaller than whitetails, so the same philosophy may apply for you in this decision.
A coyote will be about half as tall, half as thick, roughly half as long, and half as deep chested as a whitetail deer. Equally, a coyotes lungs are about half as long and half as deep, and their hearts are half as large in diameter and length. By and large about 1/4 the mass and half as large in any given dimension.
What that means: A deer's vitals at 9x are the same size in your scope as a coyotes vitals at 18x. Your aiming hold on a deer at 100yrds is visually equivalent to a coyote at 200yrds. Similarly, the target is much less forgiving, since the actual target - the vitals - is cut in half, meaning less margin for error in placement.
So when I'm hunting coyotes, I have less margin for error, and a smaller target in front of me. It's harder to see, and harder to hit. As such, I want enough magnification to manage that change. Since I do more coyote hunting than deer hunting, I get used to having the higher mag, and I tend to carry that over to my big game rifles too.
