We live 90 minutes from the San Francisco cruise terminal, so that's our perspective for this question. Choices are Ruby Princess and Carnival Miracle. Glad to have them both in SF, as we've had a great time on both ships. SF is a beautiful city to sail from.
Size: the Ruby is a bigger ship with more passengers and more crew. Neither ship felt crowded. Passenger to crew ratio is a little more favorable on the Miracle but both ships had fine service. The balcony room is a little bigger on the Miracle, so you get a sofa and cocktail table.
Value: as long as Princess keeps selling the Princess Plus package at $40 - $50 per day, Ruby Princess is a better value.
Food: MDR food and service was fine on both ships. Carnival has the edge for pool deck food with Guy's burgers, Blue Iguana tacos & breakfast burritos and the deli. Princess wins for specialty dining with two excellent speciality restaurants, both priced under $30. The Miracle has one excellent steakhouse, overpriced at $42. Carnival's made to order pizzas at Pizza Pirate were a treat, but the 24 hours paninis and snacks at Princess' International Cafe were also a plus.
Drinks: Each ship has a good coffee bar. Princess wins the wine game. Carnival is better for craft beer. Both ships have good mixed drinks, but the Miracle pool deck bars have the edge.
Live music was far better on the Ruby Princess. Both have decent theater shows. The stateroom TV's are much larger on the Ruby Princess and the on demand programming just clobbers Carnival.
Casinos were similar on both ships.
Ruby Princess makes good use of the pool deck giant screen for movies and sports; Miracle has none.
Bottom line: If you have a good attitude, there's plenty to enjoy on either ship.