I have been to all 3 areas. Santa Teresa and Mal Pais are part of the same long streach of beaches and more refers to a particular area. They are fairly remote areas so unless you and your guests are fairly adventurous I would suggest you find another more accessible area of Costa Rica. We were planning on getting married there but it was too remote for our guest to travel as we had some coming for the weekend from San Jose. It takes a few hours of driving, a ferry and you can fly but you still have to drive on some less than ideal roads. Also if you are considering getting married in the rainy season the area is virtually inaccessible because of the roads.
That being said they are beautiful areas, I was just in Santa Teresa in January and it is absolutely beautiful. A hippy surfer town would best describe the area. The hotels, beach areas and sunsets are spectacular. Actually I don't have a bad thing to say about it, it was great. They have some lovely boutique hotels in the area that could hold your wedding.
I hope this helps! Congrats and happy planning!