I have never been to Placencia although it looks beautiful but I have done many dive vacations - both land-based and liveaboards, including several Aggressor boats, and we have dived in Belize on a land-based trip on Ambergris Caye and from the now discontinued Nekton liveaboard.
It depends what you like, there are pros and cons to both land-based and liveaboard dive vacations. On a land-based trip you get to experience the local culture and enjoy more topside attractions. And that is important, especially if the weather turns bad. But diving isn't always as easy on a land-based trip, sometimes you have long boat rides to the reef and you spend a lot of time dragging your gear around. And sometimes you have to put up with boatloads of amateurs or rigid divemasters with fixed protocols - although you can usually avoid some of these issues through careful research and picking the right location, resort and dive operation.
On a liveaboard the diving is usually much easier and you get many more dive opportunities. Your cost per dive on most livaboards is usually quite cheap because the trip includes lodging and meals and many dives. You also get to visit more distant and pristine reefs. If bad weather is in the forecast the captain can often alter the itinerary to find the best options - but when the weather is really bad then being stuck on a liveaboard isn't that much fun. Some of my very best night dives have been off liveaboards because they actually wait until it is really dark (not dusk) and you aren't dumped off at the reef closest to the hotel as we have experienced on some land-based night dives.
Diving is very easy off a liveaboard. You set up your gear one time and then the crew keeps refilling your tank in place and they hose it down with fresh water. You just put on your gear and step off the boat for another great dive with lots of diving freedom. The Aggressor's motto is "eat, sleep, and dive" and that is very true. There aren't a lot of non-diving opportunities although the food is usually quite good and it is a very relaxing dive vacation. The Aggressor boats are among the nicest and most comfortable we have experienced, but living on a boat is not as comfortable as a nice hotel - in my opinion.
Although we loved Belize- I remember that the mosquitos were ferocious, just walking down the beach at night could be challenging; but when we were out on the Nekton at sea there was no problem with insects. I don't know if biting insects are a problem on Placencia but that is one factor to consider. Another thing to think about is the preferences of your traveling companions, if they are not big divers than land-based trip would have more to do.
When we do a liveaboard trip we like to extend the vacation time to spend some time on land enjoying the local people and attractions and then a week on the boat to make the most of our dive opportunities.
If you love to dive then you should try a liveaboard to see how you like it. And after you do one trip with aggressor they will offer you "dive the world" discounts (25% off per person) for selected weeks and locations that can make the trips more affordable. Most liveaboard companies offer some type of discounts for repeat customers.
Good luck with your decision!
---------- Post added June 22nd, 2014 at 10:47 AM ----------
One thing I should have mentioned in my post above is seasickness. I am usually okay on land-based boat trips out to the reef and I have been fine on many liveaboards, but if there is a deep water crossing involved (like Florida to the Bahamas) I can be very miserable during the first night onboard. It passes and soon I don't have any problems but it is something to think about if you or a diving companion are prone to seasickness. I don't remember being ill when did liveaboard diving in Belize, although I had been very ill on other Nekton and Aggressor and Explorer and other liveaboard trips. I usually start taking the triptone before boarding and I eat and drink lightly on the day of departure. If I start to feel queasy I head up on deck for the fresh air and so I can stare at the horizon - that usually helps. I have slept on deck the first night on a few trips! Just trying to share all the good and bad things about both types of dive vacations!
