We visited Placencia in October, and it is indeed a beautiful place. As a place for a wedding, I'd have to say that it would depend entirely on your taste, and the taste of those that might be travelling with you.
Placencia is much more laid back than AC, which of course also means that there is significantly less to do. It is a wonderful place to get away from it all, and there are plenty of tours, etc. We took a snorkel trip to Laughingbird Caye, which was wonderful - very good snorkeling... but if you are looking for variety in restaraunts and night life, etc. you might be better off to have the wedding in AC and the honeymoon in Placencia.
My advice would be to go check it out! You will love Belize!