In the anime and manga they kept hinting that Sasori is stronger than Deidara. Deidara even says it himself. Since I don’t think he was lieing I think he underestimated himself and overestimated Sasori. Deidara could definately beat Sasori. All he needs is some tough C3 clay and Sasori is lost. I do think it would be a very long hard fought battle. If it’s outside in a very big plain than it would be a lot easier. Deidara wins.
DR
DReager1.com 
I only think Sasori would win because thats what they said in the anime. Not because i don’t LOVE Deidara.=p
They implied it lightly, but Deidara showed his true power against Sasuke. With his ultimate Clay Attacks I think he could beat Sasori. Because he could fly high in the air and just throw bombs until Sasori loses. Sasori couldn’t reach him up there