In many ways, the original Ring Doorbell was the market leader in smart video doorbells. The second improves on a couple of areas that needed it, and adds customisation. There's no massive compelling reason to upgrade if you have the first generation.
Removable battery is a necessary addition; Camera footage is vibrant and clear; Additional mounts/fascias included for unusual doorways
Slight delay in motion/ring alerts; Needs to be fairly close to router; No in-home chime/ring without buying Chime or Chime Pro