Which water pump?

A lot of discussion of the 6 vs 8 blade water pumps in AC vs non-AC cars, resp. Some think one is better, but wonder why fewer blades if AC, how can that be better? Best explanation I have heard (maybe my idea, forgot) is that the reason is because the pulleys were different diameters. Each pump is optimal for the pulley it uses and rpm it runs at. Why would they not put the best water pump possible in all cars? Doesn't appear to be a cost difference.