Qwiss I don't think Pires would have worked on the left for Ferguson's Man Utd teams in much the same way I don't think Giggs would have worked on the left for Wenger's pre-Emirates sides. Ferguson never really knew how to use that kind of player and Wenger never had a lot of interest in your old school, touchline-hugging, flying winger.
Different players; Pires the better all around footballer, Giggs, at his best, the better pure 'winger', so to speak.
I suspect that Giggs' longevity and trophy count would skew opinions in his favour in a 'who was better' debate vs. Bobby Pires.
Of course, the answer is Pires.