I used to shoot a lot of fast twist 243 and it works great for long range. I started with a 7.75 and then 7 and 8 twists. I've shot 105 RDF, 107 SMK and 109 Hybrids and it was easy to find a load for all of them. I first used a SAAMI 243 reamer and then had JGS make me a reamer with some freebore everything worked out well. I went away from it because of the shortage of large rifle primers; I couldn't find any small primer 243 brass.
If you have everything to load 243 it will be an easy transition to heavier bullets but if you want to shoot factory ammo and handload 6 CM has the advantage. As long as you're okay with loading everything for long range 243 is good to go. I shot some 95's and 100 grain factory 243 for hunting in the fast twist and didn't have any issues with that either.