I constantly struggle with this. I use exclusively a 13-27" Harris S series but don't like it. Its heavy, to tall for proper alignment while prone. I do believe for bench or prone a 6-9" would be best for practice. For general hunting the 9-13" S would work best. I can't get myself to switch because I deal with tall grass or bushes quite often where I hunt and am forced to shoot sitting quite often. I'm thinking of experimenting with different setups like an attachment on the tripod for sitting and a 6-9" or 9-13" mounted on the gun. Whatever setup you choose practice with it a lot!!! I know my gun shoots 2MOA to the right when I shoot sitting compared to prone. Different positions will change your POI.