What size are your drains? I would use a pump that nearly maxs out your draining ability. Figure out how much your tank can drain and then purchase pumps two handle a little under your maximum draining ability (otherwise you will overflow the tank and drain the sump). Also, if you have two returns, use a seperate pump for each return.
For example, my tanks each have four 1.5" drains and two 1" returns. Each drain handles a little over 1200 gph, meaning 4800 gph drains out of the tank. I have a mag 3600 on each return (each mag is pumping about 2400 gph due to the height they are pushing the water) giving me about 4800 gph of flow. Having two pumps allows for the system to be o.k. if one pump fails. Each pump is also plugged into a seperate circuit in case one breaker fails, one pump will still be running.
Pat