Of the two I think I would go with a 6.5/284. It has a proven record for accuracy plus you can get Lapua brass for it. I have two reamers, one with a .290 neck and the other a .301 neck. With Lapua brass there's no need for turning unless you just feel better about it so I'd go with the .301 (it may be a .300 neck, I'd have to check the reamer and it's in the shop). I've built several and have never had a report of poor accuracy with that caliber.
