If I were going with a large capacity case, I would go with a 6.5 STW. Probably in the 6.5-300 wby range. Both are capable of some pretty amazing velocity. But I would think barrel life would make the 6.5-284 seem easy on a barrel. Here would be my plan, I would set a 6.5 STW up with a 9.5 inch twist and set it up primarily for 120 grain bullets, and with the right load you clould probably stabilize a 130 grain bullet. You would get unreal velocity and a little more barrel life. I don't know though how much research has been done with the 6.5s and a 9.5 inch twist though, or if you could even get one with that.