This study found that different biceps exercises affect biceps activation at various joint angles. Preacher curls are most effective at the bottom of the range of motion while standing curls and incline curls offer more activation towards the top of the movement. The takeaway? Use all variations in your training for optimal development!
Source: Oliveira, L. F., Matta, T. T., Alves, D. S., Garcia, M. A., & Vieira, T. M. (2009). Effect of the shoulder position on the biceps brachii EMG in different dumbbell curls. Journal of Sports Science & Medicine, 8(1), 24.