Grip style rather than SxS vs O/U is the determining factor. When Chris Batha measured me he gave me three LOPs: 15 3/8" for straight grip, 15 1/8" for POW grip and 14 7/8" for pistol grip. I have found that these measurements have worked out very well for me. Most O/Us are pistol grip and many SxSs are straight grip, hence the general recommendation tha SXS stocks should be longer.