I've got a rifle in each of the two calibers. The 7RUM is very impressive but might need a muzzle brake
to bring down the recoil a bit. The 264 on the other hand should be quite tame in the heavy Sendero. Mine likes the 129 gr Hornady SST and 7828 powder. You can probably expect 3300 fps with this bullet weight.
Since you are already having the 300RUM built, I would go with the 264. It is a quite capable deer round and you can make the fur fly on smaller stuff with lighter bullets.
Best of luck and enjoy your dilemma!