Lots of good choices and advice here so far, I think you need to consider a few other things too. Do you reload or will you be shooting mostly factory ammo? This is a huge consideration for obvious reasons, if you don't reload then you will have to stick to factory ammo offerings. This means sticking with more main stream options, 308, 6.5CM, 6 CM, 223, etc. Some others are available but can be tougher to get a reliable source. I had a rifle built last year in 6XC, it is pretty much a ballistic twin to the 6CM-which I should have built due to the various factory ammo options available now, and the selection of different brands and primer types for brass. But I am extremely happy with the 6mm choice, maybe slightly harder on barrels, but if you don't shoot comps you should be fine for a couple of shooting seasons on one barrel. 6.5CM also has great factory ammo options, and lots of choices when it comes to reloading components too, brass, bullets and powder are fairly easy to source now, I haven't had any issues anyways.
Good luck!