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.
Yes they both come with fast charging capability!! But its not that fast but its not slow either!! It takes just about an hour and half to completely charge the phone from 0% to 100%. Considering 4000mAh its not slow