The 9000 series and 7000 series differ mainly in the blades:
Series 9000: Up to 150,000 cutting movements per minute. This series is particularly thorough, even with longer or thicker beards.
Series 7000: Up to 90,000 cutting movements per minute. It offers a solid performance, but is less powerful than the 9000 series and is better suited to shorter stubble.
The 9000 series also offers other advantages such as a pressure sensor (for pressing the razor too hard) and the BeardAdapt sensor which measures beard density 500 times per second and automatically adjusts the power.