When comparing Planets vs Calendar, the Slant community recommends Planets for most people. In the question“What are the best custom Android Wear watch faces?” Planets is ranked 1st while Calendar is ranked 15th. The most important reason people chose Planets is:
The watchface collects NASA data and uses it to put planets in their appropriate places.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Displays the actual current positions of the planets
The watchface collects NASA data and uses it to put planets in their appropriate places.
Pro Two themes to choose from
Pro GNOME Native integration
Pro Synchronisation
It has online accounts integration.
Pro Currently in active development
GNOME Calendar is improved with every release cycle of GNOME.
Pro Simplicity
Calendar for GNOME aims to find the perfect balance between features and usability.
Cons

Con Initial spin makes it hard tell time with a quick glance
Each time checking the watch all the planets spin wildly before settling into position. It can be distracting and arguably unnecessary.
Con Eats up battery life
Eats the battery at about 3 times the rate of the standard watch face.
Con No support yet for WebCal
No support yet for WebCal, such as those offered by Facebook events.
A workaround exists. Since this application uses the same background services as Evolution, installing it and adding the WebCal calendars there, also adds them to GNOME Calendar.