Comparing Moto G5S and Mi A1

Common
Both run stock android

Differences

Screen size
Moto G5S: (5.2 inches) full HD display
Mi A1: (5.5 inches ) full HD display
Mi A1 has a slightly bigger screen. If big screen is your requirement Mi A1 is a better choice.

Processor
Moto G5S: Qualcomm^® Snapdragon™ 430 processor with an up to 1.4 GHz octa-core CPU, Adreno 505 with GPU supporting up to 450 MHz
Mi A1:Octa-core Snapdragon 625 processor, max 2.0GHz

14nm FinFET, Adreno 506 graphics 650MHz

Mi A1 has a better processor. So if you plan to play heavy games, Mi A1 is a better choice.

Battery
Moto G5S: 3000 mAh
Lasted for roughly 18 hours with heavy usage
Due to turbo charging  feature: The Moto G5S gets fully charged in roughly 60-70 minutes

Mi A1: 3080 mAh (typ)
Mi A1 lasted for roughly 15 hours with heavy usage.
Mi A1 gets fully charged in roughly 2 hours.

So going by battery Moto G5S lasts longer and charges faster. So if you don't intend to play heavy games and are more interested in a phone that lasts longer. Moto G5S is a better choice.

Sim card and Storage
Both have  Nano Sim/ Micro SD combination i.e. You can either insert 2 sim cards Or 1 sim card and 1 Micro SD card
So in this matter it's a tie.


Sources:
https://gadgets.ndtv.com/mobiles/reviews/mi-a1-review-1747540
https://gadgets.ndtv.com/mobiles/reviews/moto-g5s-plus-and-moto-g5s-review-1750813
https://www.motorola.in/products/moto-g-gen-5-special-edition
http://www.mi.com/in/mi-a1/specs/