From the airport in Miami to the port takes 15 minutes, taxi fixed rate is $24.
I prefer Eastern but take a look at all the excursions offered at the various resorts, East and West, and see which activities interest you most.
Read up on the islands listed and see which you might want to see.
Eastern- St Thomas, St Maarten, Puerto Rico, maybe private island (?which cruiseline)
Western- Jamaica, Grand Cayman,Cozumel.

Flights- I would go for whatever is cheapest and most convenient for you. I would plan on flying in the day before the cruise if possible, to avoid any delay glitches, and stay at a hotel close to the port, such as the Holiday Inn Port of Miami. It is within walking distance of restaurants, bars, live music at the Bayside Marketplace. Taxi over to the port in the morning will be around $8.