It depends on what you're looking for. If you want serious partying and nightlife, wild happy hours, a wide variety of restaurants and accomodations, I would advise Ambergris. Also, there's a wide variety of dive/snorkeling options on Ambergris. If you want peace and quiet, serious relaxing and lazing on the beach (the prettiest beach in Belize is in Placencia, in my opinion) and in a hammock, mellow, quirky locals and fellow visitors, I would recommend Placencia. Placencia has some diving and snorkeling options, but not as many and as good. Also, Placencia was hit very hard by Hurricane Iris, and it's recovery has been slow. Placencis is in a fairly remote locale, so it's difficult to get construction supplies there; also, all that rebuilding costs money. I have been to many parts of Belize on two trips, and there is charm that can be found throughout. Life moves at a slower pace: I love it there.