Regular traveller to St Lucia here. I spend all my time in the top half of the island so have no personal experience of CB but your location will depend on what you wish to do when you are there.
CB is in the South. Less travel time from the airport but the sea is rougher. Reviews seem to think this resort is of better quality than StJ. Good location if you want to hike the Pitons, visit the volcano etc.
StJ is in the North. Approx 1.15hr from the airport. It has a good family vibe, food selection is pretty good (alcohol choice/quality is not the best, but drinkable!). Staff are friendly, decent watersports. Everything is easy to access (good paths for pushchairs - not all resorts have that). I have stayed at this resort a couple of times & have never had any issues.
Just to throw another into the mix. Royalton is another that you can sometimes get in that price bracket. Quality is so much better (food, drink, beach, kids entertainment). This one is further North again, but if you were planning on staying on the resort it's worth it. It's not on BA or Virgin's books yet but it is on Tui's.
All of the resorts will offer trips out (boat trips, volcano etc).
Let me know if there's anything specific I can answer for you.