See, there ya go. We don't care which rifle you put it on!

I'd say, put it on which ever one you'll be shooting more of, and want to work on precision/long-distance. If you have 100rds of 308 and 5000rds of .223, it'd make more sense to throw it on the AR15 since you can shoot it a lot more.

And yea, I kind of agree with Fentonite. Sorta. Because I could argue that you should put it on the rifle that you are getting WORSE groups with the irons. Then, you'd have a rifle with good groupings with irons, and a rifle with potentiatlly good groupings with a scope.