In order to fulfill the basic functions of our service, the user hereby agrees to allow Xiaomi to collect, process and use personal information which shall include but not be limited to written threads, pictures, comments, replies in the Mi Community, and relevant data types listed in Xiaomi's Private Policy. By selecting "Agree", you agree to Xiaomi's Private Policy and Content Policy .
Display wise, better if you proceed to a Mi Store/Zone and compare both side-by-side to get a better idea the difference in clarity between the 720p and 1080p display.
Other factors to help you on which to choose, is mobile photography and maybe some form of gaming highly essential for you? If so, Redmi Note 5 is the better buy, it has a better set of cameras and more powerful CPU/GPU combo than Redmi S2. Also, do you need more internal storage? If so, Redmi Note 5 has a 4GB RAM / 64GB ROM version. Redmi S2 only has a 3GB RAM / 32GB ROM version and has a slightly better front-facing selfie camera. Plus Redmi Note 5 has a larger battery at 4000mAh compared to S2's 3080mAh.
All boils down to your needs.