In a general manner I wanted to compare these two islands and directly a couple of rex resorts. The islands can be very similar in price range and so a comparioson is natural.
Transfer and Immigration- Its horrific in St Lucia a twisting winding road mostly makes this around 1.5 hours. In Antigua it is 20 minutes ! If travelling with a baby this is a bonus. Immigration in St Lucia has a priority queue for those with babies, in Antigua we had to wait 2 hours baby in arms ! It was a horrid start the Antigua immigration staff could use a dressing down - we were due to get to the front of the queue and we were sent to fill out another form due to virgins poor information on the flight as we were told to only fill out a card for both us adults - our daughter needed one - virgin were very clear hence why I did not. But even so there should be a priority lane for those who need extra assistance.
On to the resorts...
Resort Facilities - From an activitiy perspective at first glance the stlucian looks to have as much going on as the halcyon however the reality is that you may (depending on you) find that some common fun activities are missing.
Note that there is a misconception that you can use both resorts no matter which you are staying in for the St Lucian - ONLY the spa is shared. In Antigua you can organise a day in the other resort. Also note that there are 2 pools at the St Lucian - a very big one a few minutes walk towards the two arge hills left when facing the beach- you can get to it via the beach its the cream coloured building that says dive shop on it. Why give us directions ? Because many people stay here without knowing it even exists. So poor are the directions and signage. This large pool has a very nice atmosphere and a swim up bar. Not always open because of dwindling guests perhaps (contact us to help you with that) if it is it is a nice place to watch the sun set behind the hills.
Spa - here is a win for the st lucian the spa is nice and the treatments are very professionally given. There is a hamam (steam room) Sauna and Jacuzzi which is FREE for all inclusive. The halcyon has a massage room which is a guest room that looks to have been converted near the bbq area.
Cricket - on the beach of course halcyon ONLY.
Rum Snorkel - halcyon ONLY - snorkel with Michael round the restaurant on the water looking for rum miniatures.
Tennis - both has though the courts in the st lucian had a rough looking surface but you could still have a game (you would need your own gear).
Beach Volleyball - Just the halcyon wins here for having a net and an organised game by Michael one of the activities guys at the halcyon ! (how come no net at the st lucian ?) Fenton the other guy who manages things along with the children's coordinator is the trio that make up the halcyon team. In the st lucian it is just Pernell Hipolyte and though he wants to do a lot the management isn't really interested in truth though you can chat to them and they make the right noises there is no action.
Pool Aerobics - Pernell does his best but cant compete with the halcyon crowd who actively drum up enthusiasm and a good group every morning even if the hotel is not so full. It also seems to be everday.
Water Sports - After paying the guys for a lesson with the activities like hobby cat or windsurf you can just take them out. In the St Lucian there seemed to be a limit on time I never got the 30 minute limit in Antigua - taking a cat out or anything for that matter takes more than 30 mins IMO.
Cost Comparison - depending on season you will have to see its not a great deal different.
Food and restaurant - the St Lucian goes to great lengths to disguise the fact that for the all inclusive guest you will always dine at the same place. You can dine at the other restaurants but you will need to effectively pay. More choices of restaurant, better food happier staff mean that hands down the halcyon cove wins. For a week its ok you can do it at the St Lucian. there is also a problem to enjoy good entertainment at the st Lucian - it is scant and not good at all. It isnt excellent at the halcyon but has a great location for it and diversity like steel drums for example.
General Criticism of Rex Resorts
They seem attentive for easy things front of house but anything difficult or at a business to business level you wont even get an email response. The have massive tunnel vision some call this focus :) this must come from senior culture and filter down. I did meet a few of them on a professional basis. The resorts are a bit dated and tired and add to the charm if you accept this otherwise you will find it irritating and uncomfortable.
The Halcyon Cove wins on Activities, Staff friendliness, Facilities, Food choice restaurants and bars, Room, Entertainment. It is easy to find your way around the halcyon and it is on a better beach.
The St Lucian wins on perhaps the island diversity and beauty but feels much more crowded also. I struggle to find a reason past the SPA why the St Lucian wins. So let me make your decision easy - if you really want some island tour diversity and better diving then choose St Lucian otherwise choose The Halcyon. The large pool in the St Lucian allows you to have space to swim in the pool in the halcyon there is just the one small pool at times. Any other resort this would win on the beach as reduit beach is excellent so dont make a choice based on this.
Also the St Lucian seems to go out of its way to be annoying at times like having to go get a parasol from reception and then get the water guys to put it in the ground for you. Any normal resort has them already on the beach. The St Lucian also has cruise ship 'grade' beds set up auto and in a specific area. They can get a nice beachside BBQ but not us !
Boat traffic in BOTH resorts means you need to swim in the ropes otherwise you take great danger upo yourself. For a keen open water swimmer this is a disappointment - hard to control but it could at least be better.
More on the Comparison Antigua and St Lucia