There is one good reason to use SPD-SL over Keo, but hardly anybody needs it; the pivot location makes it possible to have the auxiliary straps for track sprinters actually pull the cleat hook more closed as the strap gets pulled. Other than that, it's just personal preference; Shimano are cheap and strong but heavy, Look are light but wear out quicker.
There is one good reason to use SPD-SL over Keo, but hardly anybody needs it; the pivot location makes it possible to have the auxiliary straps for track sprinters actually pull the cleat hook more closed as the strap gets pulled. Other than that, it's just personal preference; Shimano are cheap and strong but heavy, Look are light but wear out quicker.