The displays on both phones are great, but if you're primarily looking to consume multimedia, the K6 Power edges the Redmi 3S Prime out.
However, if battery life is your main consideration, then the Redmi 3S Prime is the phone to get. In all the phones I used last year, there wasn't one that came close to the battery life of the Redmi 3S Prime. MIUI's aggressive memory management combined with a huge battery means you'll easily get two days of battery even if you're a heavy user.
I have tried and tested almost many phones from different brands. And my expert opinion is to go for Lenovo. Lenovo is a far reputed and bigger company not only for smart phones but also for laptops, desktops, tabs, power banks and other accessories. Lenovo launches less phones per year as compared to MI but all phones are nice in a compact package and has value for money and also fetches a good resale value.