Xiaomi Mi 5X seems to be the best choice out of the three given choices as it offers a better overall specification like better processor configuration, More RAM, bigger display and battery life etc. in comparison to Xiaomi Mi5 and Xiaomi Mi A1.
If the specs of mi 5x will be same as it shown on other websites the camera of both phone will have same 12 +12 mp,so i can say there will be no difference in at all but some little improvement we can expect from them such as some new modes and little bit bug will fix
Xiaomi Mi A1 seems to be better option,It has premium looking metallic body and impressive cameras make it a great choice. The configuration is strong too, which means gaming and multitasking are going to be a pleasure on this device.Xiaomi Mi A1 vs Xiaomi Mi 5X