Deidara has said it himself that Sasori is alot stronger than him. In my opinion Deidara should really be much stronger than Sasori, i mean Deidara managed to beat Gaara, who was a real beast. He also had a very even fight with Sasuke, who managed to beat Orochimaru pretty easily. I´m not saying that Sasori isn´t strong and in fact Sasori is one of the strongest members of Akatsuki. However Chiyo said that Sasori isn´t so good at analyzing his opponents(unlike Deidara).
So, who is the stronger one, Sasori or Deidara? You decide