Little more complicated than that. People with very wide sit bones tend to use a B68 as their road oriented saddle. A too narrow saddle doesn't support their sit bones and is very painful. A B17 is a little bit narrower, and then there are Brooks saddles that are narrower yet like the B17N and the Swift.
But! Someone with sit bones that aren't too wide for a B17 can find a B68 very useful if they're on a quite upright bike. These may have suspended seatposts, and then Wallingford's advice makes a lot of sense. (or they might not, and still be helpful, because a leather saddle has a little bit of natural give)
Keep in mind that the width of your sit bones may not have anything to do with the apparent width of your hips. My hips are quite wide for a female, but my B17 is Just Right. Some bike shops offer a sit bone measuring service (Trek and Specialized in particular). That might be helpful for you. Also, some stock bikes come with a B17. Test riding one of those can be *very* useful.