What country are you going to?
I shoot a .300 win for almost any hunt around the world because I HAVE arrived at a destination with my rifle and no ammunition. If you are going to Kyrgyzstan, for example, it is illegal for an outfitter to provide a camp rifle for foreigners, so keep this in mind when planning. A .300 PRC might be better on paper, but you aren't likely to find emergency ammo for any "new" caliber like this in Asia.
Ibex are tough, resilient creatures that like to climb into the most inhospitable country when wounded, so my advice is to not wound one. Use a bonded bullet that won't explode at high velocities. Practice out to 800 yards regularly and it will greatly enhance your confidence at 400. I have shot nine ibex, and eight of those were at ranges from 80 to 500 yards, for what that's worth.