Do you hunt with grandad's 6mm? If so, then the current-production .243 is a little redundant, since they are really ballistic twins. If you don't actually hunt with it, then keeping the .243 makes sense, because you'll have one to use, that has more common ammo, and you'll have grandad's rifle in the safe, not getting worn in the field.
I had the choice this summer of getting a really nice, slightly used Winchester in 6mm or a brand new Savage in .243 for about the same money. I got a .243 because of the commonly available brass and commercial ammo. Just seemed to me to make more sense - I can get ammo for it anywhere, and when I do buy commercial ammo for it, it costs a lot less than 6mm.
I almost bought the 6mm, though, because the truth is I was intending to mostly reload for my rifle no matter which caliber I bought, and the 6mm cases seem to be just a little bit better design for that. But I have found, so far, that the .243 is very easy to load for. We'll have to see how long the brass lasts. I think I made the better choice here, because of the cost savings on virgin brass, once fired brass, and commercial ammo are all huge.