It really depends on the twist rate of your gun(s). A 7mm 180gr berger VLD has a BC way over what is used in .243, 6.5mm, or even most .30 calibers. If high BC matters to you, the 7mm (as long as you have a barrel that can stabilize these heavy bullets) is probably one of the best choices out there. But, on a calm day at 500 yards, the BC difference probably wont be a deal-breaker. A .243 will be slightly less expensive to shoot, have less recoil, and a somewhat flatter trajectory, but will lose effectiveness past 500 yards, especially with any sort of wind. The 180gr 7mm is still going strong at 1000 yards, with still enough energy to be effective on game. It will also buck the wind significantly better at longer ranges. If 500 yards is what you'll be shooting most at, go with the .243, if you want to shoot past 500 yards with regularity, then go with a 7mm mag.
Note: Before you guys jump on me, I'm not saying a .243 isn't capable of shooting accurately past 500 yards, I'm just saying that the 7mm will be significantly better in this regard, especially in conditions that aren't perfect.
Just my 2 cents