Screen Protection: The Xiaomi Mi A1 comes with 2.5D Curved Gorilla Glass 3. However, the Mi 5x doesn't feature any Gorilla Glass Protection.
Android UI: "Made by Xiaomi, Designed by Google" The Mi A1 comes with Pure Stock Android with two pre-installed Xiaomi Application. Xiaomi and Google plans to update the Mi A1 to Android 8.0 Oreo by the end of 2017 and to Android P whenever it gets launched. The Mi 5x on the other hand comes with MIUI 8 based on Android Nougat.
Variants: You get two variants with the Mi 5x 4GB RAM and 32GB ROM, 4GB RAM and 64GB ROM. The Mi A1 on the other hand features only 4GB RAM and 64GB ROM.
Availability: The Mi 5X is China exclusive, however you can import that device from Gearbest. The Mi A1, thanks to Google's Android One Project will be available in tons of countries.
That said, the Mi A1 is the Mi 5x with Google's Back, you can find the Xiaomi Mi A1 pricing, specs and availability details here.