Just to make sure you dont fall in this common trap: "lower flow rate means more time to exchange heat", this is totally wrong.
Heat exchange rate depends on exchange surface. More flow rate, more surface. In theory you can get better heat transfer by have the highest possible flow rate.
In reality very powerful pumps tend to build lots of heat, due to their own motor power (Joule effect here). But those pumps are so expensive and so big... it's not the point here in computer watercooling. Just make sure your pump aint crap. What you want is *decent* flow rate and yet a silent pump. thats it
