Water pump impeller styles?

By design the 8 blade pushes more coolant.
But it also cavitates lower in the RPM's because of it.

I run the 8 because 90% of my normal driving is at lower RPM's on tight little streets and small hills. (My Dart is my service vehicle)

As far as coolant water cools best unless a wetter product is used, but also keep in mind that it boils at a lower temp also.