If you like St Kitts, i think you will LOVE Aruba
I do Aruba every summer. Last February i went to St Kitts for a week, and while i had a relaxing time and thought it was nice, it didnt come close to Aruba. Although i will say, it reminded me of Aruba the first time i went, 30 years ago, but its totally different today. And the Aruban beaches kill the St Kitts beaches
One thing, i know there are a lot of fans of All Inclusive. For me, it doesnt make sense to do AI in Aruba, becasue there are so many great restaurants on the island.