My .243 is highly accurate. I have 4350 loads with 100 grain bullets that chronograph a hair under 3,000 fps. controlled expansion or solid bullets would give clean through and through shots. it's a beautiful, lovely package, bolt rifle, .243, 3-9 scope, and moderate range shots at moderately sized white tails. It would even be legal for elk. (not that I agree with something like that for elk).
7 mm will give you a slightly heavier bullet, slightly lower velocity, slightly worse trajectory, but if you can find a great gun in 7mm 08, I would prefer having the extra bullet weight on impact. It will give deeper penetration and a more damaging wound, and there would be very little ballistic difference.
Check the ballistics charts. If you may be shooting over 2-300 yards, I'd choose the best, flattest shooting round, otherwise, I'd go 7mm. Practically speaking, it will only hold a modest edge over the .243/6mm, but it is, however, superior.