Ann, all the itineraries look relatively comparable, with Caribbean islands and days at sea. There is good sightseeing, water sports and shopping in all of those ports.
Ft. Lauderdale and Miami are about a half hour apart by car. You can go from Ft. Lauderdale to Miami in more than enough time. In fact, you're likely to get to Miami before they start letting people on the ship. A taxi would cost about $65 and will hold 4 people, although luggage space would be tight. You could wait for a taxi van, which has more room. A transfer is inconvenient since you have to pick it up from the airport. With the cost of getting to the airport and paying for the shuttle for all of you, a taxi is probably cheaper.
As for the length of a first timer's cruise, ordinarily I would say that 15 days is too long. But since you're coming all the way from Australia, and since I know you'll all have a great time on the amazing ships you're looking at, it could work very well.
An alternative may be to take one cruise, and spend a week on land, sightseeing in the Miami/Ft. Lauderdale area, or going up to Orlando (about 3 hours away by car) to go to Disney World. Or head down to the Florida Keys.
I don't think you could make a bad choice.