I think it depends on what you'd call, "Better."
Pegasi:>smaller so less wind resistance>lighter and probably more maneuverable>capable of high speeds>butt symbols can allow for greater specificity in terms of flight
Griffons:>greater wingspan so more powerful>aquiline morphology better suited for flight>above point probable cause for greater flight endurance as well
In all I'd say that griffons are probably more specified for power and endurance while pegasi are more all-rounders with an edge on speed an maneuverability.
Ultimately they're figuratively and literally in different weight classes and like comparing short range interceptors to escort fighters.