I think most people would agree the diving off Placencia is a little better than that off of Ambergris Caye, but the drawback is the reef is a lot farther away, 15 miles or so. Of course the best diving in Belize is even farther out, around the atolls.
Whale sharks are off Placencia in the late spring, April to June or so.
As to lodging, Placencia has much more of a variety in the budget end, somewhat less at the higher end, although several of the small resorts (Robert's Grove, Rum Point, Kitty's, Nautical Inn, etc.) there are excellent. There's more nightlife on Ambergris Caye, but there are some good restaurants and a few bars on the Placencia peninsula.
Belize First Magazine
(and Fodor's) http://www.turq.com/belizefirst/