Dual speed pumps are more designed for spa use only and rarely serve any purpose on a pool/spa combination. About the only time a dual speed motor would be installed on a pool/spa combination is if your were going to continue using the spa after the pool is winterized. In that case, the filter NEEDS to run whenever temps get near freezing and continue to run until the temps rise above freezing. In this part of the country that could be weeks at a time. Then, a low speed pump would be more economical. Other than that, I'd stay with a single speed pump. In addition, it'll be less expensive to replace when it dies!