[ViBiker]

Both have their advantages and disadvantages!

DD
Advantages:
Quiet - Almost absolutely noiseless)
Efficient - +90% efficiency
Less expensive than "geared"
Dependable - 1 moving part
Excellent heat dispersion
Responds well to over volting - 36V hub (23+mph) pumped up to 48V (~30mph) seems to have no heat issues)

Disadvantages:
Large
Drag - with motor disengaged there is a slight drag

Geared
Advantages:
Nearly silent operation
Efficient - +90% efficiency
"Proven", durable, gear reduction and freewheel technology
Small size - usually smaller than sprocket cluster - difficult to notice
Inconsequential "drag" when disengaged - nearly 0%
Better "low end" torque than DD

Disadvantages:
Usually more expensive than DD
More complex than DD - more chance of a problem
Less heat dispersion surface area - danger of overheat when over volted or over Amped