Both are a good choice for pronghorn. Since you don't have much time I would go with what is available in the model of gun you like best. Odds are that will be the 6.5 Creedmoor because it has been a popular cartridge the last few years and often what has been kept in stock on the shelf. Another good option to consider is a 6mm Creedmoor if you can find it in the model of gun you like on short notice. I'll be shooting 115 gr bergers next week out of a 6mm creedmoor at a Wyoming pronghorn. The 115 gr bergers are shooting about 3050 fps in my 26" Creedmoor and around 2930 fps from my 22" AR10 6mm Creedmoor. Hornady 103gr ELD X is a widely available factory ammo that is flat shooting and very effective pronghorn bullet out of a 6mm creedmoor.