I've sailed on both ships, and like both very much. But there are differences between the ships, which might influence which you should sail.
The Sun has the better cabins, with small refrigerators and more shelf space. The Spirit has better public rooms and decor in general, its Asian decor is unique.
There are a few restaurant differences, with the Sun having the better Italian restaurant, Il Adiago, while the Spirit uses a part of the buffet for the La Trattoria. The Sun also has a Las Rambas Tapas Bar, while the Spirit has the Bier Garten, neither ship have both.
The ships also have different itineraries this winter.
Sun New Orleans - 5:30 PM
Mon At Sea - -
Tue Costa Maya 8:00 AM 6:00 PM
Wed Santo Tomas De Castilla 8:00 AM 6:00 PM
Thu Belize City 8:00 AM 6:00 PM
Fri Cozumel 8:00 AM 5:00 PM
Sat At Sea - -
Sun New Orleans 8:00 AM -
Sat Miami - 4:00 PM
Sun At Sea - -
Mon Roatan 8:00 AM 5:00 PM
Tue Belize City 8:00 AM 5:00 PM
Wed Cozumel 8:00 AM 5:00 PM
Thu At Sea - -
Fri Great Stirrup Cay 9:00 AM 6:00 PM
Sat Miami 8:00 AM -
I would choose by itinerary, does either ship go to where you haven't been before. Unless you plan to use the refrigerator in the cabin as a wet bar, neither ship has much of an advantage over the other. The only other differences I can visualize effecting the choice, are food preferences and homeports.
Whichever one you choose, you'll have a great time. I would choose both if I could. With that in mind, choose the Sun this year, and the Spirit next year. The Sun will reposition to South America the following winter, which might make it too far away.