I'm going with Dei Dei-kun. I think he has more of an advantage with dodging Mei's lava release since he can stay in the air, our of the way. And Mei might have trouble preventing all the clay bombs from going near her... Just my opinion though, I think it would be a really interesting and tough battle.
if deidara goes in the air mei could spit lava on his flying clay thing and blow him up lol. id pick mei deidara is too goofy for me to rank over a kage, and mei is pretty bad ass with her acid mist which would eat up bombs before they got to her, and all her differant natures i think she takes it.
I'll say Deidara cause Mei lacks the ability to keep Deidara from getting on the ground. It's easy for Deidara to get up in the air and fight to his advantage with just some C1 bombs. After he's in the air, Mei hardly stands a chance. Corrosive Mist might have an effect on Deidara's bombs but Deidara can send a barrage of bombs to Mei. Chances are she won't have enough mist or speed to stop Deidara's attacks.