I shouldn't be including the 260 calibre to the context of the orig. question which asks which "6mm is best choice".
In comparing all the 6mm's I simply favor the 243Ackley because it doesnt require alot of brass forming exercises and is easy to reload.
My 243 Ackley with 1:10 and 75-90 gr. bullets produces the highest velocities for flat long range shooting. This is a super accurate calibre !
Sorry to hijack the thread but...I had thought of going the .243 route when I was building a rifle last year but wanted something with a little longer barrel life. My .257 Roberts AI pushes 75gr V-Max over 3700fps with ease and should last a life time. I've only shot one coyote with it so far but it messed it up pretty good.