RE: Barrel length- .257 Roberts and 7mm-08
You won't see much difference in performance between 22" bbl and 24" bbl with a case this size.
Personally i'd pick the 22", because to me it's a better balance between "performance", and haveing more "muzzle weight" than a 20" bbl will give. More importantly a 22" bbl will get the rifle "balanced" properly in your hands. Assumeing this rifle will be built on the lighter side?? It would be if it was mine.
I like the Roberts, but the 7-08 is a better all around cartridge!!