Great question! Cleats are a personal decision. Some people swear by SPD pedals. Others swear by Looks. What I can tell you from my experience is that SPD has two major drawbacks. First, I have had athletes who have gotten mud in their cleats from the transition area and then they could not clip in. Second, from an injury prevention standpoint, I have seen cases of plantar fasciitis. The cleat is much smaller and can cause cramping of the bottom of the foot. Looks pedals are wider and use a larger portion of the bottom of your foot and can minimize the cramping.
However if you are aware of these issues (or don’t have them), I don’t really think it matters. If there was a strong advantage of one over the other from a speed perspective, everyone would have only one brand of cleat and pedals.