Hi, I'm building my first AR with 6 ARC. I was wandering what buffer length to use. My plan was to use a carbine but I have seen several guys using rifle length but not sure of the reasons. Any advice on this would be appreciated.

Welcome aboard Kaos - are you still tracking Maxwell Smart- lol!!!
My recommendation is to use the carbine buffer system, that's what I do. IMO the grendel and the 6 Arc are both better suited to carbine length springs and buffer weights. I also tend to use butt stocks that have an adjustable cheek weld, for my scope use. Mixing and matching, as well as have a standard set of cross-usable spare parts, are also benefits of this set up.
These 2 calibers have a larger variation in usable bullet weights and powders/ charges, so the wide variety of options in carbine length give you room to adjust.
My 2 cents anyway!

Thanks, I've got a lot to learn!

Agreed. Carbine tube due to variety of springs and buffers to fine tune things if needed. So far my Arcs run well on standard or H1 buffers.

Enjoy!