In the adventure of planning this trip we have knocked out Grand Cayman and repleaced it with St. Lucia. Which one is better St. Lucia or Antigua? And how is Coconut Bay Resort in St. Lucia for a family?

No one island is "better" than another they are just different. it depends on what YOU mean by "better" - "better" for what????

Perhaps if you gave us some indication of what exactly you are seeking in a vacation experience it will make it easier to give you meaningful recommendations. All we know from this and you previous post is you are seeking something "adventurous" and some nightlife. It would help if you defined "adventurous" for us.

By adventurous we mean good sightseeing and excursions.

New online but really enjoy the membership. We vote for the later. See search results and we went 6 months ago. Safe, cheap, clean.

There's good sightseeing and excursions on both St. Lucia and Antigua - again what you'll see/do is not necessarily better, just different.

St. Lucia is much larger than Antigua so by definition will have "more" to see. Plenty of lush rain forests to hike, mountains, waterfalls, volcanos, sulphur fields, hot springs, fortresses etc plus all the usual waterspouts. Antigua is not as lush but has many more (365) beaches. Activities center around some hiking and lots of waterspouts.

Once again Antigua is the winner if you are looking for beaches and activities. St. Lucia is nice but is very hilly and many areas are too remote to reach.
Antigua is famous for beaches and sailing, especially sailing week. You said you coming for a 16 year olds birthday so activity wise choose Antigua.

Thank you all so much! Everyone has been very helpful!