go for flow velocity.... it is much more important than the actual volume being pumped.... the higher the velocity, the faster the blocks volume is getting replaced with fresh cold liquid.... also the more turbulent the flow will be, a very good thing.
I always say, massive flow-rate only means a lot of water went through without doing nothing to cool the CPU. flow rate needs to be converted into flow velocity and turbulence. If the pump can not handle the pressure, don't blame the block, get a better pump....
MO. And I live by it.... my blocks are all low volume high velocity designs.