Which water pump?

Just get a lifetime warranty model. They are all the same, you are only paying for the warranty. The impeller was designed for the pulley system it operated on, hence the different blade count. I dont think one will outflow the other and if you think about it, they both will be pumping through the same small stat opening so hi-flow is a gimmick. A new water pump wont help cooling unless your old impeller was rotted out or off the shaft. A simple fan shroud is a free cooling upgrade.