Nothing wrong with a standard .243. Started shooting one in matches in 2005 and had no problem fitting them in AICS mags with 115s. I still have a .243 in the safe and shoot the Hornady 105 BTHP and loaded .020" off the lands they are only 2.810" OAL. Get them at 3080fps with 42.6grns of H4350.

My main match rifle is a 6.5 Creedmoor so I have plenty of brass to make 6mm Creedmoor so my next 6mm barrel will be a 6mm Creedmoor as I have a lot of friends who shoot it and really like it. It will allow you to get a slightly shorter OAL and good velocities with 105+ weight bullets.

Either 6mm is a good choice for long range shooting when paired with a proper twisted barrel for the bullets used.