I also reccommend not having it as a duel use rifle. As said, a serious competitive rifle will be giving you a weight issue when using it for hunting. Been there, done that - they get heavy. I'd also be a bit leary about taking my competitive level rilfe in the field in case the unfortunate whoops were to occur. Nothing like banging that competition level scope.
As for chambering...as it's to be a competitive rifle, I'm sure for the utmost in performnace you'll handload right? That said, either would suffice and give darn equal results. Were you not loading I'd say go with the more available .300WM