Open for twist and barrel brands and general input.
Twist rates.
For varmints the 12 is the fastest I would want to run. The 12 allows for the best accuracy doesn't overspin the thin varmint jackets to bullet failure. For yotes which isn't my chosen path I would think the 12 would work as well, but the normal thought would probably drive a person to the 10 twist. Which is again not as bad as the now shooters want to have at 8 twist or faster, solely because of the PRS guys in their desire for long for caliber bullet, and the hype. This has caused the offerings of twist rates to be slimmed down greatly reducing options.
If I had to pick a offering for just Yotes it would probably be no faster than a 10 twist. A 9 if there just simply isn't any other choice, while the 12 would on paper work with say a 87gr Seirra Game King, it might need a faster twist (10) depending on how cold it is where your hunting. For me personally being in a warm climate such as Arkansas I don't have to worry so much about that.
The other issue will be the throat/leade area. If you desire to shoot solely 105gr lengths or longer this will not pose a problem in the 6mm ARC or 6x45. Although off the top of my head I want to say the throat/leade distance is way more forgiving (shorter) in a longer setup in the throat/leade area in the 6x45 than the 6mm ARC.
Velocity the ARC will win. Accuracy is a toss up between the two. your mentioned distance of a 300 yard cap the X45 case will do the job.
Component wise the availability of 5.56/223 will absolutely rule the roost over the Grendel/ x39mm case of the ARC.
Sourcing barrels. Looking at the prefits of Shilen the 6mm ARC is a 1-7 twist (>$600.00), then for liberty which is actually done by Satern barrels. The differance between the liberty / satern branding is how the bore is cut liberty is button pulled the satern is rifle point cut. Liberty has the ARC in a 7.5" twist at about half the cost of Shilen (<$400.00). Liberty does offer a 6x45 20" 1-8 Twist for $255.00. Ballistics Advantage which is the most affordable is less than the cost of the Liberty (@<$200.00 less shipping). Reports of the BA offerings are mixed, and that is honestly based on the reviewer point of view and expectations. There are a bunch of differing offerings other than the ones I posted based on what you posted and my thoughts of your stated direction.
As we can see the offerings have been driven to the faster offering in twist rates. While I have not actually provided guidance to your pursuit, maybe the background noise of my recommendations and statement will assist you in choosing between the two calibers. Dragging us back to the early Henry Ford days you can have any color as long as it's black.
