Creedmoor. Performance wise they're all but identical, but the creedmoor is shorter and fits in the magazine better, which will allow you to seat the bullet out longer over time. The Brass argument used to be the main one used in favor of .260, but at this point I think that argument swings the other direction. Lapua, Norma/nosler, Alpha, Peterson, Hornady, and some others that I can't think of all make brass for 6.5 Creedmoor now. High-quality factory match and hunting ammo for 6.5 has become cheap and plentiful, too.