Capacity is king, semi-autos win this big time. Revolvers are slow to reload exacerbating the capacity differences. Revolvers are harder to learn to shoot, and reload effectively; for a fixed amount of training and practice time invested nearly all shooters will be more proficient with a semi-auto than a revolver. Take a step back and look at what police are actually using, they did not make that decision in a vacuum. Of the departments that issue department sidearms are there any departments in the US that still issue revolvers? I would guess that is a very small number of departments. How many departments that allow the officer to select his own sidearm still allow them to choose revolvers? I would bet that number is also pretty small.