You won't get as much speed from the 6mm Creed as the 243win. A bit better barrel life in the Creed case. 243win AI is a moderate bump over the straight 243win, but less barrel life still. Since you're getting a custom barrel, you have access to whatever twist you like, so you're not stuck with the typical 1:9-1:10" of the 243.

I agree - the 6.5 Creed really wasn't anything which turned my crank. What it does have, which I appreciate, as an advantage over the 6mm version is barrel life. For a hunting rifle, you won't likely be terrible concerned with barrel life. I have a Ruger Precision Rifle in 6.5 creed, and another in 6 creed, plus that Seekins Havak in 6 creed. Nothing magic, just a short action cartridge which can push a ~105-110grn pill to ~3000fps... I started out with Hornady brass and WLP's, converted over to Lapua brass with BR4's. The 108 ELD's shoot incredibly well, but I get a little tighter with the 105 Hybrids. I expect either bullet would do the job nicely for your objective of pronghorn.

In a custom rifle where you can pick the twist, you probably don't gain anything by going 6 creed over 243win other than barrel life. Either will have the appropriate twist, and you'll be able to pick a throat to let you run the heavier bullets in either. I will admit, I did order a new Savage barrel for one of my Striker's, to convert it from 243win to 6 creed, but really only because I am shooting a lot more 6 creed right now than 243win.

In 243win, I run Lapua Palma brass to give me small primers, necked down. Currently, you're stuck doing the exact same thing in 6 creed, as Lapua isn't making 6 brass yet - we have to neck down 6.5 creed brass. The hornady 6 creed brass is very consistent, but it HAS to be annealed every other loading, if not every loading. My SD's went through the roof on my 3x firing - not handy to realize at a match!