Recently bought the Redmi 3 Pro and loved the decision to get one. I'd noticed that the Redmi 3S is meant to be slightly better and spotted it was on sale at the moment.
All the specs are in the link, it's a top phone and I understand that MIUI 8 (OS) I'd being rolled out which is good if you read the forums on Mi phones.
I have a Three contract and 4g works fine but do check your providers frequency to ensure you will get 4g on this phone. Most won't have issues but just a heads up.
Xiaomi Redmi 3S Android 6.0 5.0 inch 4G Smartphone Qualcomm Snapdragon 430 Octa Core 1.4GHz 3GB RAM 32GB ROM Fingerprint Scanner 13.0MP Rear Camera
Display: 5.0 inch 1280 x 720 HD screen
CPU: Qualcomm Snapdragon 430 Octa Core 1.4GHz
System: Android 6.0
RAM + ROM: 3GB RAM + 32GB ROM
Max Storage Expansion: 128GB
Camera: Back camera 13.0MP + front camera 5.0MP
Feature: Fingerprint Scanner, GPS, A-GPS
SIM Card: Dual SIM dual standby, one is micro SIM card, the other is nano SIM card