How do you feel about one terrific scope for both rifles? Mount a 20MOA pic rail on both, get a set of Badger Ordinance rings and bounce it between both rifles. Zero it on the .308 (you'll shoot it more) the first time you move it to the .338 make notes on how many tenths of a mil it was moved vertically and horizontally to zero it to the .338. Every time you move it, it will be within less than .5 Mil of where it should be, so shoot one round at 100 yards to be sure.
I've had this scope on loan from Bushnell to test and review for two months and it is one of the best optics I have ever looked through. The glass clarity is amazing and the reticle is actually very nice, it is not too busy. It will be getting extremely high marks as far as I'm concerned and I am probably going to have to buy it!http://bushnell.com/all-products/rifle-scopes/elite-tactical/ers-3-5-21x-50mm