The Royal Guard don't really have any statements suggesting one is stronger than the other, though Pitou definitely got more focus, played a larger role and has better feats than the others.
To me, Youpi always seemed like the more combat oriented Royal Guard, with Pouf as the strategist and Pitou a mix between the two, and Youpi's nen ability is great in 1v1 combat whereas Pitou's is kinda shit and useless aside from Terpsichora.
Though, having said that, it's honestly doubtful that the Royal Guard are even capable of harming each other since we see Pitou face tank an attack from Meruem intended to kill her and Pouf does something similar later against an even stronger Meruem, so a fight between them could literally just go on forever given their near infinite aura.