Mi4C is better in every aspect except RN3 has metal body and bigger battery capacity.

- Much better display (1 : 1600 contrast ratio  )
- Better performance and more power efficient than Helio X10. Cortex A57 (Snapdragon 808) > Cortex A53 (Helio x10)
- Better at graphically heavy tasks. Adreno 418 > Power VR6200. VR6200 is about the same as G2 (Adreno 330)
- Better camera...Mi4C is the first to use the recently announced Sony Exmor RS IMX258 with phase detection focus and 192 focus point.

Redmi devices never had Sony sensors for camera if not mistaken. Most of them using less capable Samsung BSI sensor to cut cost and to separate with Mi flagship series.

If you don't need big screen and don't fancy all metal design, Mi4C is the better phone overall.