They are both outstanding, but both are limited by the factory 1:9ish twist rates. I would hesitate to get either one with anything slower than 1:8 twist if you want to shoot above ~0.300 G7 BC bullets.
I have both, in nearly identical rifles (Savage LA, X-Caliber 1:8 5R barrels), and the 264 WM has a moderate velocity advantage, and noticeably less recoil, when comparing the same B.C. bullets.
24" 264 Win Mag, 147 ELDM (~0.350 G7 BC) = 3050 fps
26" 7 Rem Mag, 175 ELDX (~0.350 G7 BC) = 2950 fps
The 7 RM does have a couple of bullet options that have significantly higher B.C. than any 264 bullets (e.g. 190 A-Tip, 180 ELDM, 195 EOL).
For me, the decision would be made based on hunting scenario. Up to deer, 264WM is about the best there is. If elk, moose, or walrus is on the menu past 400 yds, go with the 7 Rem Mag.