I think Sasori is being underestimated. I think Sasori is the better ninja. He was more intelligent. He slacked off in his fight with Chiyo and Deidara showed his full strength against Sasuke. I think Deidara would loose because Sasori could make it a battle of speed. Sasori's 100 puppets could prevent Sasori from making huge attacks because he would always be pressing him.
I also think it would be very hard for Deidara to avoid every one of Sasori's attacks. All of his puppets and their weapons had that deadly poison on them and as we saw with Sakura it only takes a scratch to prove deadly. I think Deidara is at a disadvantage in this battle because his style requires time to be completely effective and Sasori can be straight to the point.
1. Sasori has his fake body (hiruko)
2. he had the Kazekage along with his Iron Sand techniques
3. the 100+ puppet techniques
4. every weapon was poison based. either dipped in poison (cutting weapons) or gas
5. he had all the elements at his disposal
6. his real puppet-ized body
7. YEARS of experience.
8. Deidara kind of feared him, and respected him to a degree
Plus if none of that helps his case, we can always look at the fact that Deidara said Sasori was stronger.
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