Any of the power ranges you mentioned are generally fine, depending on your shooting.
Some people make a big deal about power over 10x. Heck, about 20 years ago most people found 4x to be perfect for big game, and the "radicals" used to use 6x (then considered for varmints) for big game.
By today's standards, the 3x-9x is king. But do you know what? I have used power as high as 16x on deer at 200 yards and it doesn't seem to bother me. My only concern is when there are other animals nearby because the FOV is smaller. Other than that, it doesn't bother me.
Keep in mind that pronghorns are realatively small. As such, you can, generally speaking, go up in power a bit. The other thing is, it also depends on your shooting distance. If shots are over 200 yards, or even 300 yards, then a higher power scope may come in handy.
I tend to prefer higher power scopes, but perhaps that partly because that's what my relatively "younger generation" is supposed to believe.
Zachary