Mine is a .257 Roberts.
I don't see a real strong practical reason to choose one over the other and what reasons there are seem to mostly offset.
Unless you want Nosler, .257 factory ammo and reloading brass is pretty scarce. Mine likes 120 grain partitions and 110 grain accubonds ... haven't seen either in months. So far it shows no love for 100 or 115 grain partitions.
With those bullets it is, IMHO, fractionally more effective than the .243, however, I don't work in fractions, if I need more than a .243 will do I'm not stopping at .257 caliber, I'm going for a .270 with 150s, a 7mm with 160s, a .30 with 165s or 180s .. etc.
I've seen a little bit of WW .243 brass lately but 6 months ago it was nearly as hard to come by as .257 brass.
It is possible the .257 will have a little more barrel life. Doesn't matter. Barrel life in a rifle bought to hunt game, not shoot varmints or paper, is measured in lifetimes. A non-factor.
You might consider .260 or 7mm-08 if you can find one. Both have been discontinued in the Montana. You might find one on a shelf somewhere, otherwise it's a "build" proposition. If you are going to build, you might also want to think about a .22-250 std or AI with a fast twist. Its a notion that's been percolating in the back o' my cobweb collection for a while.
Tom
