Up to 600 yards, there's no reason IMO to go with any wildcats or throat-burning chamberings.
Those few extra FPS make a difference at longer ranges- but most shooters would be hard put to be able to recognize the ballistic differences of any 6.5 chambering at that distance.
Suggest, that since you are new to long-range shooting, go with the .260 if you want a 6.5.
It is widely accepted as having the best compromise between barrel life, cost to shoot, and performance. And it'll be a looong time before you can outshoot a match-grade rifle in that caliber out to 1000 yards if you choose.
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