Re: 3-9X40 or 10x42 for mid-long range .257
The 3-9 Conquest's glass may edge the 3-9 SS by a bit, but not the 10XHD. Though unless stand hunting at long range only I just can't see hunting with a fixed 10X so my choice for hunting would be the 3-9. The glass is very decent, but the turrets, reticle, range of travel, etc, are what make it so nice to use for close/mid/long range all around use in a fairly compact package.
Though like many here, I like higher powers as well if he doesn't mind packing something a bit bigger.