Sasori wins. For one, deidara outright called him stronger.
Secondly, he's a hard counter for deidara.
The Iron sand is stronger than gaaras sand which could easily tank c3, and thus c2 and c1 as well. c4 is useless since sasori doesnt have lungs, and c0 is a suicide move.
Gaara was able to take a limb off deidara, and at that point he was weaker than Rasa, who in turn was weaker than the Third Kazekage. Sasori killed the Third Kazekage, and possesses him. Thats 2 mid to high kage level fighters right there.
Sasoris sand is poisoned, and seeing how an inferior version of that sand could tag deidara, he should be able to tag him as well and kill him