Re: Undecided 243 Win or 6.5 Creed
I'd also look into the .260 if I were you... It's in the 6.5mm family and is a ballistic twin to the 6.5 Creed, but if you ever intend to reload, the market for reloading the .260 is pretty well developed in terms of brass, dies offerings. The main difference between the .260 and the 6.5 Creed is that the 6.5 Creed has a different mouth angle so (correct me if I'm wrong, I don't yet reload/shoot either .260 or 6.5 Creed) the pressure will be lower for the 6.5 Creed than for the .260 with the same load. You can also reload the 6.5 Creed, FWIW, but you either have to deal with the meager brass offerings out there or size donor brass in addition to ordering custom dies IIRC.
Also in the 6.5mm family is the 6.5x47 Lapua, but it has too many downsides (at least in my opinion) for the avaerage shooter. The case capacity is smaller than the .260 or 6.5 Creed, so you're dealing with higher pressures and hotter loads to get the same results, and you get a bit of a barrel burner. The upside (or downside?) is that you pretty much have to use Lapua brass, so the hot loads won't make you go out and buy new brass every 4 loads.
Of course, alot of this is based off of the hint I picked up in your post that you might like to reload sometime. If you don't intend on reloading anytime in the near future, get the 6.5 Creedmoor and don't look back. The Hornady match ammo is just too good, and it's decently priced.
Edit: I just wanted to point out that the two calibers you offered are in two different "families" - the .243 is a 6mm and the 6.5 Creedmoor is a 6.5mm. That said, if you want a straight up comparison, look at the 6mm Creedmoor. I personally would go with the 6.5mm over 6mm, but it depends on a number of factors that mainly hinge on what you are doing in terms of shooting and loading and what you want to do. IIRC, the .260 will be less of a barrel burner than the .243, have slightly more drop than the .243, but with the larger bullets it will buck the wind more.