Pro Screen fits more content
The Amoled display provides rich colors are true blacks which is even more important on a watch (likely it will be displaying black very frequently). From a functionality standpoint, a square display fits more content on it, especially when the entire UI is based on rectangular cards.
Pro Bio sensor
The bio sensor measures your pulse, and keeps track of your relaxation level throughout the day (a lower pulse means you are more relaxed). It's neat to see what part of the day you are stressed out during, and if you notice patterns during high stress times you can change that.
Con Poor battery life
During a heavy day (with ambient mode enabled for the convience), the Zen Watch's battery could clunk out after just 14 hours. You could put it into flight mode to stretch the battery time to about 20 hours, to keep the always-on mode enabled but then it isn't performing the functions of a smartwatch anymore.