BR brass is easy to find for sale. but me, being cheap and wanting an alternative source, would go with the 6mmPCC. Even though brass 7.62x39 isn't super easy to find, it's out there and the forming dies from x39 to pcc are affordable and have apparently good results. I am of the belief that if you can make use of all the available 6mm bullets, and match the nessecary velocity to take advantage of the bullets accuracy, their is no reason one will be superior to the other, except of course the quality of the rifle itself. i would choose the 6mm that has the easiest to obtain compenent for you and where you live. and also choose a 6mm that will even allow you to cross-match compenent for rifles that you already reload for. The obvious easiest 6mm in terms of compenets is of cousre, the 6x45 using .223 brass. i am not sure if the 6x45 has the same potential as the BR, but i have heard reports it can be just as good. But, i would think if that was the case, we would be seeing everyone using it, but it may be very appropriate for 2-300 yards like you said you wanted. If you were doing 500+ yards the .243 has super available components as well, although the Dasher has proven to be the winner in the 600-1000 yard arena.
you could just go to one of the BR matches in your area, see what folks are running and discuss with them the pros and cons of each. yo will at least get to see the competition you are going to be up against.
My dream right now is a 6mmx54R. I WILL get there, just not sure when.
