I've stayed at both Rosewood resorts. As a hotel we much preferred Little Dix Bay because it was smaller than Caneel. The beach there is wonderful and we regretted the fact that they built a pool (ditto the pool at Curtain Bluff in Antigua). As soon as you have a pool you get a different sort of customer.
However, Caneel has the advantage of having several beaches and many of them were truly world class. But the hotel's a bit big for our tastes.
We've not been to the FS in Nevis, though we did once stay on Nevis before the hotel was built and before it wrecked the landscape. Nevis is most definitely not a big rnesort sort of place. You need to stay in a plantation-style property of about 20 rooms.
Frankly, I'm not sure if the Caribbean has anywhere truly outstanding these days, apart from that Aman joint in the T&C. I quite fancy St Barts.