I believe Deidara wins.
-If he gets on his bird immediately before the Mizukage can summon the clam, he could fly out of range of the mist the clam emits, and rain down explosives until he dies.
-Even if he gets caught, the DC of his bombs(especially c3 or c4) would destroy the clam, and end him.(I doubt the Hozuki's are immune to something that destroys things from the molecular level)
- Lastly, if he has intel, using clones and hiding underground can counter the Mizukage's genjutsu temporarily, and I believe his clones could be used to plant the land mines, which the Mizukage can't detect, and stepping on one can kill him.
So, Deidara should take both scenarios IMO.