[pushkins]

You can ask that very same question from 100 people and get a 75/25 split.
For my money I'd never install both ridge or roof vents and gable vents on the same roof space. Ridge vents and roof vents (provided the roof vent is high on the roof) with correct soffit venting act like a chimney, drawing cool air in through the soffit forcing the hot air out the vents giving the air "direction".
When you add in a gable vent that can also be effected by outside wind the air inside the attic becomes "unstable" this means that it has no direction, most evident when you use a smoke stick, the smoke tumbles over itself inside the attic. When you use the same smoke stick in an attic with soffit and ridge vents only the smoke streams upward to wards the vents.