1600 MHz
Apple MacBook Air (2017) 13.3" Intel Core i7 2.2GHz / 8GB / 128GB
2133 MHz
Apple MacBook Pro (2017) 13" with Touch Bar and Touch ID / Intel Core i5 3.1GHz / 8GB / 256GB
It can support faster memory, which will give quicker system performance.
RAM speed: is this important to you?