I have owned several 264 Win Mags and several 270's over the years.
As has already been stated, to capitalize on the 264's potential, one should have at least a 26" barrel.
However, with today's powders in that 26" barrel, the 264 will approach 3300 with a 140 grain bullet, and believe me, that shoots very flat over 5-600 yards!!
Any 264 I have owned has been wonderfully accurate, and the last one I had built with a 26" Pac-Nor would drive three 140 Partitions onto one ragged hole at 200 yards if I did my part.
I have shot Black Bear, Muleys, Whitetails, Elk & Moose with the 264, and never felt the need for a bigger diameter bullet, nor for the 160 grain RN bullets, which, IMHO, go counter to the idea of the 264 as a long range hunting chambering.
I have a tremendous amount of respect for the 270, but the 264 has more oomph and of course, more "panache" than has the 270.
Buy the 264, you'll enjoy it immensely. Even with a 24" tube, it is a fabulous performer.
BTW, that Leupold 6x42 is a beautiful piece of glass.....I have several on hunting rifles. Took my longest successful shot ever with just those optics.
Regards, Eagleye.