The Spectra S1 and Spectra S2 are both excellent breast pumps, but they do have some key differences. The main difference lies in their power source - the S1 has a built-in rechargeable battery, making it portable and convenient for on-the-go pumping, while the S2 needs to be plugged into an outlet. Additionally, the S1 offers a massage mode that mimics a baby's natural sucking pattern, providing added comfort and stimulation. On the other hand, the S2 does not have this feature. Both pumps have adjustable suction levels and customizable settings to cater to individual needs. Ultimately, your choice will depend on your specific pumping requirements and lifestyle preferences.