It really depends on what you like to do. Do you like to shop? Do you like to visit ruins or see local culture? Do you like to lay on the beach and bask in the sun? Do you like snorkeling and water sports. Do you like more time on the ship?
If you like to shop then Eastern is better.
If you are into Local culture and Mayan ruins then go with Western.
If you want all around water sports, snorkeling, diving and beach days then western is a better itinerary. If you want more time on the ship then Eastern will usually give you more sea days.
The choice is yours.