Either would do well for you...
Brass might be a little easier to acquire for the 260 (even if you were necking up .243 win cases)...
Three guys who shoot our long range course here in SW Virginia are using 260's, and all three shoot the same load: 142 grain Sierra Matchking and 38.0 grains of.... Varget. No kidding. That load rocks in the 260. 2800 fps from 24 inch barrels, so it's not too hot... just an accurate, easy load that works.
Bullet selection between the two cartridges is about a draw... great long range bullets in both 6.5 and 7mm.
Barrel life... I'd give the edge to the 260 there, for sure. But the .284 would not be bad either.
The 6.5/284 gives on average about 100 fps over what you can get from a 260, maybe a little more... but not a lot. The 260 for all intents and purposes will do what the 6.5/284 will do.
For taking down game past 600 yards, however... the 284's heavier 7mm bullets would be better, having more energy for the job.
Either way you go, you'll have a winner. Both are great cartridges.
Dan