This is really a comparison of a unilateral movement (dumbbell curl) with a bilateral exercise (barbell curl). It must be made clear; there is no ‘most effective’ movement – both have benefits and drawbacks.
For example, compared to dumbbells, a barbell will allow for maximal overload – the heaviest weight possible that can be lifted. This is crucial in building maximum muscle. Conversely, barbell work is limited in its application of correcting muscle imbalances such as differences in limb strength.
One of the main reasons for the lack of progress in many bodybuilders physiques’ is not workout frequency, intensity, or duration. Lack of progress can often be attributed to muscle and physique imbalances that manifest from poor training program design.
The correction and prevention of muscle imbalances are crucial to force-production, reducing the risk of injury and the ability to apply maximum overload. As I said, barbells are limited in application in this area.
Although less overload is usually involved with the dumbbell variation of a particular exercise, they are a most effective tool for the correction and prevention of imbalances that can cause training plateaus.
As you can see, each type of movement provides unique advantages that the other cannot. Therefore, I’d select and incorporate both barbell and dumbbell work into your training.
And finally, there is the efficiency equation. Bilateral movements (barbell curls) are far more efficient than unilateral exercises (alternating dumbbell curls). You expend almost twice the energy and time with unilateral movements to achieve the same overload.
As you can see, each type of movement provides unique advantages that the other cannot. Therefore I’d select and incorporate both barbell and dumbbell work into your Max-OT program.
I recommend starting your bicep workout with barbell curls. After you warm-up, 3 sets of heavy barbell curls will stimulate growth most efficiently. Follow that with 2 sets of dumbbell curls and a set or two of dumbbell hammers curls.
With the dumbbell curls, curl both arms at the same time. Avoid alternate curls to increase the efficiency of the exercise. This will take a little practice but you’ll get the hang of it quickly.