What is the difference between parallel blade volume damper and opposed blade volume damper? which is better?

As per SMACNA HVAC system duct design Page6.4 :

Volume control dampers are manufactured with either "opposed" blades or "parallel" blades. When partially closed, the parallel bladed damper diverts the air-stream to the side of the duct. This results in a non-uniform velocity profile beyond the damper, and flow to branch ducts close to the downstream side may be seriously affected (See Figure6-4).

The use of an opposed blade damper is recommended when volume control is required at the fan outlet.

The parallel blade damper use to adjust air volume only when airstream deflection acceptable

Parallel blade dampers are usually selected for volume control from wide open to% of wide open. A relatively large control arm swing provides sensitive control through a relatively small change in air volume. Parallel blade dampers also offer the best first-cost selection for simple open-closed damper requirements. 

Opposed Blade- Opposed blade dampers are selected for applications requiring volume control over a broad range, from wide open to% of wide open, as the control arm swing is more proportional to the dampered effect.