ET Deidara is a more fun character imo - he's got a kit that synergizes well and he can fit in quite a few teams.

Kakuzu is a bit more niche, you kinda hafta use him in Akatsuki teams to be useful and he's a bit clunkier to fit into teams. ET Deidara would be my choice, but if you're a big Kakuzu fan then I say go for him.