If you reload, the 308 eats a larger pie out of available powders of suitable burn rate and is rarely picky about any of em (think using the same powders as a 223). If you don't reload, ammo for the 308 across the board is also going to be cheaper.
I've never seen steel cased 270's or basement priced FMJ ball for the 270.
If going the cast route, a 308 is a lot easier to get success with the least amount of effort versus most 270's as well.
And if they make an AR in 270, I'm not aware of it.
