The WS2815 2121 Pixel LED Strip is high-density 200leds/m, it can reach to a much better animal lighting effect. WS2811 has two data cables, which ensures that even one led is not working, it won't afffect the whole strip's working, it's a good choice to make big strip screen. 

Advantage:

1.DC12V operating voltage instead of DC5V, the LED strip has lower voltage drop at long distances.

2.Backup data line from previous LED so that a defective LED has no influence on the rest of the LEDs. Two defective WS2815 LEDs behind each other have influence, however that chance is minimal of course. 

3.Less tight timing of the reset signal so that the WS2815 LED can work better with low-end microcontrollers

4.External capacitor to ensure a stable performance and better heat dissipation.