The two are very different. Ambergris Caye is far more developed in terms of tourism infrastructure, number of hotels and restaurants, shops, etc. Placencia is developing pretty rapidly, but in a lot of ways, other than some new hotels, it looks much like it did 10 years ago. There are hardly any stores worthy of the name on the Placencia peninsula and most things, even fresh produce, have to be brought in from Dangriga.
Placencia is still very much a seasonal destination for tourist. The late summer and fall can be dead, whereas San Pedro slows down some but never like Placencia.
Ambergris Caye has a resident population of about 4500. Placencia village has around 500 people, Seine Bight around 900, and there are a few hundred other people living up and down the peninsula. (Across the lagoon in the Independence/Mango Creek there are several thousand other folks.) Lots more gringo expats in San Pedro than Placencia.
Placencia is a Creole village, a little funky. Seine Bight is Garifuna, in culture shock, not yet really recovered from Hurricane Iris. San Pedro was settled mostly by Mestizos originally from Mexico.
There's a totally different feel to the two areas. Ambergris Caye has a laid-back Caribbean resort island atmosphere. Placencia, at the end of a 25-mile dirt road, is more like ... well, it's just different.
As to real estate, both are pretty active. Real estate really has been driving development in Placencia for several years, especially at the north end of the peninsula.
Ambergris Caye real estate is pricier, though not by much.
The bargains in both places are long gone, unless you compare prices to those along the coast of the U.S., and then you might think it's still a deal.