I for one do not buy Sasori being stronger than Deidara. Feats triumph statements, especially when there is/are contradiction(s) between the two.
One example can be poison vs C4 Karuda/Garuda.
Sasori's best poison takes approximately two to three days to kill someone. In the process of killing someone, the poison inflicts a form of paralysis and immense pain as it destroys muscles (due to being metal-based in nature). On top of that, it requires you getting a single scratch, and the likelihood of being scratched is determined by how fast the attack is and the speed at which the victim dodges the attack, making it avoidable in varying difficulties by adjusting how fast the two variables (speed of the attack and the person dodging) are. On top of that, if there is a skilled, powerful doctor that is on the level of Tsunade or Sakura, the poison can easily be nullified/lessened to a less deadly state before the cure is administered if a cure is not available. If a cure is available, then the poison is cured right off the bat. We saw this happen when Sakura was treating Kankuro after he had a run in with Sasori and I remember the moment pretty well (it was that gruesome in my opinion because I am squeamish and I don't like looking at entrails).
Deidara's C4 has two stages. The first stage is being the giant clay figurine of Deidara and the second stage being the nano-bombs. Deidara so far has only used C4 when he knows someone is in range of it. Considering how explosions work, the nano-bombs would be dispersed relatively quickly throughout the air within the given range of the attack. Once inhaled, Deidara can detonate the bombs that are present in his victim's/victims' bloodstream(s) and cause cellular-level explosions, disintegrating his victim(s) within the time frame of seconds to a minute with using a lightning style attack on yourself being the only way to escape that fate.
As for destructive power, at most (in the canon), we saw Sasori's fight destroy the inside of a cave. This is not counting how he killed people, this is destroying things like buildings and the landscape. Deidara, on the other hand, has been shown capable of destroying the entire Sand Village in his fight with Gaara by using one C3 bomb. On top of that, there is the fact that C0 has a range of approximately 20 kilometers, which is far greater than Sasori's range with his puppets and iron sand.
In the end, Deidara was just blowing smoke by saying Sasori was stronger than himself.
