The high guard
The high guard keeps both gloves close to the temples with elbows tucked to protect the ribs, forming a compact shield that is simple to teach and effective against straight punches. Its cost is that the lead hand is less available for jabbing and vision is partially obstructed, so fighters using it typically rely on head movement and footwork to close distance before committing, rather than using the guard as an offensive platform in its own right.
The shoulder roll
The shoulder roll uses a turned stance and a raised lead shoulder to deflect incoming punches by rotating the torso at the moment of impact, letting the rear hand stay low and ready to counter immediately. It rewards fighters with strong core rotation and precise timing, since misreading a punch's trajectory leaves little backup protection, which is why it is taught less often than simpler guards and tends to be associated with a small number of especially technical fighters.
Why no style wins outright
Comparing guards in the abstract misses the point, because each one suits particular attributes: reach, reflexes, preferred range and the type of opponent typically faced. A tall fighter with a long reach gains less from a shoulder roll built for close-range countering, while a shorter fighter who needs to close distance often finds a high guard's protection more useful during the approach. Style selection is therefore a fit problem, not a search for a single best answer.