If you are considering varmint hunting with it the .243 is really the only viable option to me. Almost every manufacturer makes a rifle chambered for it, same goes with ammo. And there are way more choices in ammo as well from light varmint rounds to heavier big game bullets. The 6mm might look good on paper, but in real life it sure doesn't give you very many options. Especially if you don't reload.

My opinion anyway

Paul