6.5-284 for sure. If you only wanted to shoot to 600 at your local range for fun the 260 is choice, but... if you are going to 1000 in benchrest matches the 6.5-284 exists for a reason. With a custom chambered barrel you have lots of options. 140 gr VLD bullets seated into the rifling don't need much throat (ask Mr. Berger) and single shot followers make more sense in 14+ lb rifles. 6.5-284 for sure.

"If all the good luck and all the bad luck I've had were put together, I reckon it'd make the biggest damned pile of luck in the world." Charlie Goodnight