The 260 case since your not having to neck down from the 308 is little easier and if you got in a bind if 260AI is headspace right you can fire factory ammo in the 260AI chamber.
http://www.mansonreamers.com/Instruc...y%20Gaging.pdf
I've always build my Ackleys using the parent case and I like to use new brass so I've got pretty good history from the start.
Well good luck