Talk to your agent (hopefully they are a Cruise Specialist) and perhaps they've been to each of the ports in both itineraries and they can provide information on the ports and what to do.
But it really depends on what you like to do (sightseeing, shopping, snorkeling, scuba diving, etc, etc.). Both the Eastern and Western Caribbean have alot to offer and both are nice in their own ways.
Definitely don't want to book directly with the cruise line because they won't be able to offer any information to help you with your decision.
But if you're celebrating your 20th anniversary, you should make it special and maybe even look at other cruise lines that can provide a bit more upscale experience without an upscale price. Again, talk to you agent about the options available.
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I prefer 8 days, over 7, and Ft. Lauderdale over Miami. The ports don't matter too much to me, but as noted above, if you prefer water sports, and drinking, Western might be better. If you prefer sight seeing, and shopping, Eastern might be better.