I am a big 6.5 fan and actually own both a 26 nosler and 264 win. Here's my loads that I settled on and experience with the same components (26" barrel, nosler brass, federal 215m primers, retumbo powder):
264:
- 140gr berger elite hunters, 72.0gr of Retumbo, 3,217 fps - shoots consistently in the .3's
- 156gr berger elite hunters, 68.5gr of Retumbo, 3,103 fps - shoots consistently in the .2's
26 Nosler:
- 140gr Nosler Accubonds, 74.8gr of Retumbo, 3,265fps - shoots in the .1's
One thing that I did notice is that around 76.8gr in the 26 Nosler, you could get another 100 fps, but I had a pretty good plunger/ejector mark on the brass and 2 more grains of powder to only get 100 fps didn't seem like a good tradeoff.
If you were going to pick one, I'd go with the 264 win mag - assuming you are using similar components like I did. The real world gains in the 26 Nosler is not worth it in my opinion for the decrease in barrel life. I have another buddy who loaded up the 140gr accubond in his 264 and his speeds were within 30 fps, with 6 grains less powder - same barrel length, same powder (26", Retumbo). The 264 is just more efficient, bottom line. You start getting diminishing returns after about 70-72gr of powder.
BUT, if you want a 26 Nosler, it works great! As long as you use it as it's intended - it's a hunting caliber. I'm always a proponent for shooting faster, as long as the accuracy is still there - it's just easier to shoot at moderate ranges with less hold over or adjustment in a real world hunting scenario.
Either way, make a call and run with it and have fun!