We have been to both islands and stayed at a Sandals at each. Antigua is better!
Good points: Nice resort, good variety of food choices -- 4 restaruants to choose from including a pizzeria. Service was good, polite and attentive. We opted for an oceantront room which was beautifully decorated. To watch the sunset from the area in back of the beach where the tables are with a Green Russian was a great way to end the afternoon. The landscaping is nice.
The entertainment is pretty good and there is something a little different every night.

Bad points to St Lucia Regency: This paticular location has some hills to climb to get to certain parts of the resort even though there are stairs and a shuttle. There is a fair bit of walking if you are in the upper area.

This resort is also a long ride from the airport. The shuttle is a minivan with a number of other couples and the luggage.

The worst thing: I had many, many bug bites. It took a long time for them to disappear.

Additionally, there were many people that deserved an apology or compensation for some of the things we went through.

My Conclusion:
I would not go back to another Sandals in St Lucia.

I hope this helps you.