Imagine two different cartridges loaded with the same, long bullet to a magazine length of 2.82". One has the bullet seated deep into the case to fit in the mag, leaving the bullet .2" off the lands with no room for adjustment, the bullet ogive may also be below the case mouth. The same bullet can be seated in the other case all the way to the lands, or deeper, wherever best accuracy is found, all at mag length or less. Both cases have about the same net case capacity loaded to mag length. Which case is preferable? It's not really a zero sum game, for factory rifles and limited mag lengths, one is better and one is worse.
OP,
Is this rifle for hunting, or just for range shooting? If you plan to use it for hunting, what animals do you plan to shoot at what ranges? If it's just for targets and steel, even the lowly .223 can be a lot of fun at 600 yds and in as long as the wind isn't too bad.
If you want to do some hunting and range shooting, a 6.5mm or 6mm would be a good bet. I'd never suggest anyone buy a .260 over a 6.5 Creedmoor, you'd have to look much harder to find rifles/ammo, and wind up with an inferior package for your extra effort.
In 6mms, I would personally lean towards the 6mm Creedmoor, it's a better design for shooting heavy bullets out of the box, but unlike in the case of the 6.5s, the 6mm Creedmoor is not nearly as popular (yet?) as the .243. If you do go with a .243, be sure to get one with a decent twist rate. You couldn't give me another .243 with a 10 twist, been there and done that, it really gimps the cartridge for heavy bullet use.