wow, I'd say it just wouldn't matter if you are going to be sightseeing, dining, swimming, gambling, relaxing and just chilling out! I don't think you could go wrong with either. I've seen St. Kitts but not close up. It is off the Atlantic side I believe and looks nice from the road we were on. Haven't been to Curacao. Beware of the monkeys on St. Kitts though. They are prevalent in the Rain Forest areas.
We took a tour to the old Sugar Plantation and when we got out of the jeep, the driver started looking around and scared all of us half to death thinking there could be muggers or something. He said to take our purses and any valuables with us. Then later we found that the monkeys steal everything in site
Enjoy your trip wherever you decide to go and by the way welcome to the site. Please keep us informed regarding this trip and any others you may have. We love to share information. It's a give and get site as someone mentioned a couple of days ago.
I hope someone will chime in that has actually been to one or both of these properties. Safe travels