I found a little snippet comparing 6.5 Grendel to 6 ARC. I will have to do my own research and see what kind of ballistic differences there are. As they say in the video though, with 6.5 bullets your main bullet selection kind of max out around the mid 120 grain range whereas the ARC is designed from the ground up for all 6mm bullets without really encountering magazine length restrictions.

It might be just academic but that's my specialty. LOL

Also if you just want to shoot (and reload) I did a quick search on Ammoseek and 6mm bullets start out about right at 1/2 the cost of a 6.5 bullet. Premium 6mm projectiles are still cheaper per bullet (with better BC etc) than standard 120ish grain 6.5mm bullets.