FLL is about 20 miles from the Port of Miami. MIA, on the other hand, is 6 miles. So what you save in airfare may be much less than what you spend to get to the port from FLL.
In a later post, I'll cover transportation options from either airport to the port, but in general, FLL is only a better option when the flight is more than $50 less than one into Miami AND you are traveling with a group. If you're traveling alone, it would need to be more than $100 less to save you money. Shuttles from FLL are available but many require at least two people to make a reservation. (You then have the added step of finding/waiting for your shuttle.)
If your friends are flying in at different times, or if you don't want the extra bother of reserving shuttles, taxis are the way to go. Taxis from FLL are about $75 + tip. If you've got 3-4 people sharing a ride, that could still make the FLL fare better. On the other hand, taxi fares from MIA are around $20.
As for me, I've got fare trackers set up on kayak.com so that if a flight happens to drop below $200, it's mine. And if it doesn't, I'll probably book in March.
Good luck booking!