It really depends on the buffs you get from both. Typically i see boomkins with staves and trees with daggers and an OH but you need to add up stats of dagger and OH and compare to the staff, determine stat priority (aka haste vs crit rating), and pick which one is best for you
I see boomkin and Restos on my server with main hand and offhand.
As far as my experience goes if you compare a epic staff to an epic main hand and offhand....
The MH and OH is way better because you get more from them. The only druids that, in my opinion, should use a staff is feral specc. Staves for dps/tank are more abundant then those for restos/boomkins.