What's the difference between the Spectra S1 and S2?

The S1 has a rechargeable battery and the S2 is plug-in only. Otherwise the motors, suction range (up to 270 mmHg), and accessory ecosystem are essentially identical. Reviewers who pump at home or near an outlet often save money with the S2; those who want to pump in a car, on the couch, or at work without finding an outlet lean toward the S1.