Are you a reloader? Practicing to become proficient at longer ranges will eat up quite a bit of ammo, so take that into consideration as well. Of the 2 choices you listed I'd go with the 243. Components are readily available, and reasonably priced. If you don't reload, there's lots of choices for factory ammo. I'd also look into the 308, barrel life is excellent and ammo/components are also readily available and relatively inexpensive.
I'm sure you're going to get lots of replies and probably the same amount of caliber selections.
