Both ships are the same age, both designed for freestyle dining, have about the same restaurant choices so I would say choose the one with an itinerary you like, or go for the lesser priced of the two. I do want to mention, they are not sister ships.
We have been on the Sun twice, and not the Star yet (5 plus weeks) one thing the Star does have that is missing from the Sun: Blue Lagoon dining (not a decider for us, but we do like Blue Lagoon) The Sun has Las Rambles for tapas and I don't think the Star has the same. They may have tapas in the Mexican restaurant just like the Jewel does, but it isn't the same as Las Rambles by any means.
Where are you thinking of going on one of these ships? If it's a choice between the two of them for Alaska, I'd recommend the Sun. Sailing out of Vancouver, you do much more of the inside passage than if you sail from Seattle.
If it's a choice between the Mexican Riviera and the Caribbean -- do both (not at once time, obviously).