That chart shows some of the difference. 6mm ARC was designed to take advantage of the higher BC 6mm projectiles like the 108ELDM. 6mm MAX apparently has issues with the "longer" 6mm projectiles (like the 108ELDM) and seems physically constrained to 80-100 grain projectiles (yes it will shoot the 55s and others). Note the twist - that is why ARC uses 7.5 twist and MAX uses 8 twist.
It is a moot point until 6mm MAX barrels, ammo, brass, etc. becomes commonly available. Again, each is a niche cartridge and the 6mm ARC may be the "best" at a wider range of applications. 6mm MAX may dominate the Quantified Performance type competition.