The former allows for four inputs, while the latter works as an eight-way D-pad that makes diagonal inputs easier.
Easy to swap out, watch a YouTube video on how to install them, and it will take you about 15 minutes to finish the installment.
Compatibility:
Elite Wireless Controller Series 2 (model: 1797)
What's in the box
1x Faceted D Pad
1x Standard Dpad
(Important: The Controller and any other parts not included.)