Placencia is quite different from Ambergris Caye. It's much less developed, though it's undergoing a big real estate boom right now. No condotels yet, mostly very small inns and hotels. Not nearly as many restaurants or bars. Think Ambergris Caye in 1985. It's a bit more difficult getting up and down the peninsula, since it's 16 miles long and there's are no water taxis and the shuttle van is on again/off again. Taxis are expensive. Unlike San Pedro, Placencia tends to slow down a great deal in the off-season, with a number of hotels and restaurants closing, especially in September. More no-see-ums than on Ambergris, mosquitoes not usually a problem.
For more information, you might look at one of my books -- Guide to Mainland Belize, Fodor's Belize & Guatemala Guide, UpClose Central America, etc.
Belize First Magazine http://www.belizefirst.com/