schmidty, the AR configuration for the 6.8 is great for varmints. The only drawback is that you cannot go over 115gr bullets. The magazine in the AR is too short to allow the 130gr bullets. The superb ammo in the Hornady 110 match Amax or Vmax plastic tip (26.00 per 20rd) is an excellent choice for many things. Watch close,, as Hornady has 2 choices and the same price but one is much more accurate then the other. But if you want to go lighter weight in the bullet selection the price goes up. But if you roll your own, the door widens. The 6.8 will recoil less and is lighter in weight. The 243 is going to be heavier and recoil more. You can hot rod a 243 a little more than the 6.8 but then I have not tried doing that in an AR configuration. The next thing you need to consider is hand-loading or not.
If you roll your own then you can go with the 6.8 and give yourself a few more choices than the factory selections. If you don't re-load and don't plan on starting then I suggest going with the 243. This cartridge has been around a long time. Plenty on the shelves and prices are better than the 6.8SPC ...The last thing to consider is do you want to wade through the muck when it comes to the choices of chambers and barrel twists. Do not misunderstand I have a 6.8 in AR and it is great. But, be sure and get the newer 6.8 SPC2 chamber and if you can find it get an 11 1/2 twist. Now I am sure this is going to start a few things with other members but so be it. It was not meant to!
This has been part of what hurt the 6.8 as far as being more popular. The first chambers were not quite correct, and therefore when ammo makers such as Silver State Amory, had too much pressure for the chambers that Remington originally came out with. The twist was not correct either causing the 6.8 not to be as accurate that it was actually capable of. Now, don't get me wrong,, I have an old rifle that is the original chamber and I still get 1" at 100 (5shots) and 4"-6" at 300 if I do my part. And if not tooo windy LOL It is an AR,, and so far they have not been quite as accurate as a bolt gun. But now that I have a few of them I won't part with them. Put a good scope on one and I even enjoy the Acog,,,,and you can't go wrong. A good scope helps any AR as it does with a bolt gun.