Sentient,

A two speed pump will nearly always save you money assuming you run the pump on low speed most of the time. If you go with the same HP pump but in two speeds, the lower speed will have about half the flow as high speed so the turnover is twice as long. This means you will have to run it twice as long on low speed than high speed for the same filtration. The power usage at low speed for some pumps is about 30% of high speed (60% reduction). But since your running the pump twice as long, you will save close to 30% of the power than high speed.

Even without the rebate, the savings would pay for the pump in about a year.