At least you've narrowed your ship search to a cruiseline, so thats a start. Here is an article that may also help you decide:
Personally I prefer the Southern, then Eastern and last western. More water activities on the western, while better shopping on the Eastern and Southern. Its really a personal preference, but since this will be your first cruise, it will all be new and you can do a different itinary on the next :-)
Here is also another link with great tips and information for a first cruise, hope this helps. Do let us know what ship you finally decide on, enjoy!
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