Re: which 6mm ???
My requirements were almost identical and after months of posts in several forums and correspondence with knowledgable people mine will be a straight 243 Win.
6XC is smaller case than I want. Fireforming put me off many alternatives and easily available components are important here. 243 case was my choice; you may look at Joe Hendriks' 6mm Comp Match (CM) or the 243 BRK and 6mm Super-X. All are 243 based. If not for my requirements the Lapua case seems a great alternative. If I lived in the US I may have tried the 6mm CM.
I chose the easy route on reamers and dies too.