It depends what you want to emphasize. A wider stance will allow greater glute-ham activation at lesser knee flexion but less potential maximal knee flexion (due to leverage….you can't go as low as in a narrower stance because you'd tip over). A shoulder width stance will allow you to go as low as flexibility will allow and the lower you go the greater the glute-hip activation. A recent study showed that the stance has little effect on quad activation but does influence muscle activity on the medial thigh (wider = more activation) and glute (as above). These results however were comparing activation at equivalent depths, and as noted above, a narrower stance will permit greater depth and thus equal or greater hip-glute activation. Personally, I think high-bar, full squats with a shoulder width stance are best due to their balance of overall strength development, strength development in extreme ranges of motion, flexibility development, and a more "athletic" or "sport-like" body positioning. Having said that, all squat stances can have a place in a good program.