A good rule of thumb is to buy only as much twist as absolutely necessary to stabilize your intended bullet. A 12 twist will eliminate using heavier bullets. An 8 twist will shoot both, but with slightly less accuracy or consistency with lighter bullets, as long as you don't have wind to worry about.

FWIW I built a 6mmBR with 28 inch Criterion bull barrel in 8 twist on a Savage 12FV action and it shoots tiny groups using 107SMK's, but then I don't compete with it. If you're just hobby shooting like me, ultimate accuracy is not as big a draw as when you're spending lots of coin to compete. JMOP.

My advice: get a big, long, fat barrel, load it with 105 to 108 grs bullets and shoot the snot out of it.