I haven't been to Antigua, but I was in St Kitts last Wednesday . I did the Grand Tour of St Kitts tour with Thenford Gray's Island Excursion . It was excellent!
Regarding the beaches in St Kitts, South Friars beach at the Shipwreck bar was a very, very narrow piece of beach with the beach chairs practically in the water. It was not my idea of a good beach. On the Atlantic side, close to Timothy Hill, the beaches were wider and flater. But we were told the water is tougher and is prone to rip currents. On our island tour I didn't see any other beach areas other than the two I mentioned.
hope this helps you some in your decisions.