Personally I like both Placencia and Ambergris Caye, but it's a matter of personal choice. I'd probably say do both.
One difference is that San Pedro is a neat little town, and Ambergris Caye has quite a number of shops, restaurants, bars, etc. whereas the Placencia peninsula is more a string of small resorts on the beach with only two little villages (and one of them, Seine Bight, is of interest mostly to those interested in Garifuna culture).
FYI Turtle Inn has been bought by Francis Ford Coppola who also owns Blancaneaux Lodge in the Mountain Pine Ridge. Turtle Inn is reopening Dec. 15 with some upgrading as Blancaneaux's Turtle Inn.
Green Parrot, under new improved ownership, is fine, though a bit out of the way if you don't have a car.
Belize First Magazine http://www.turq.com/belizefirst/