Turks & Caicos or St John??

I’m taking my 17 y/o niece on a trip in June for her graduation. She lives in TN and wants a beach trip – we settled on the Caribbean. She is not very adventurous and mostly wants to visit beaches, shop & go to cool spots. We will likely do a sunset cruise or something like that. I have already been to Aruba, Mexico, Cuba, Jamaica, Bahamas, Puerto Rico, & Costa Rica.

I have done a ton of research and decided on either Turks & Caicos or St Thomas & St John. I see SO MANY great things about St John, but I’m wondering if it will be boring for her, especially for a week. I don’t like the super touristy areas and cruise ports or all-inclusives. I prefer to be somewhere that has beautiful areas & fun things to do but isn’t too overcrowded. Also need somewhere that won’t break the bank. What are your thoughts?