Can't speak to St. Croix but:
Aruba is great. 85 degrees and sunny every day. The ultimate in Caribbean-blue beaches. Marriott property there is very nice - right on the beach. Downside is that Aruba is very commercialized and the beaches near the Marriott are super crowded and jam packed with people, stuff, business ventures, boats, palapas, etc. Really quite junked up if you ask me. It is not secluded at least not in that area. Food and shopping in Aruba are world class. Island is safe and prosperous. It is very windy there. Very windy. Did I mention the wind? Also it's a flat desert terrain and not the pretty green rainforest and mountains you find elsewhere in the Caribbean though. But the beaches are flat-out fantastic despite the proliferation of people and stuff - calm light aqua waters as far as the eye can see. Feb will be full of NY and northeast-coast people who are fun-loving, friendly and loud (they sound like Fran Drescher).
pal13, Aruba has a great airport, by Caribbean standards anyway. Americans can clear US Customs in Aruba before boarding the plane. It works well and saves time. But of course no customs at all to clear for St. Croix, at least for those of us in the US.
I have no basis of comparison as I have not been to Aruba. That said, I cannot recommend St. Croix and the Carambola highly enough. I've stayed there for a week on points in March for the past three years (and this is a reminder to book for 2017). St. Croix is a laid back island, with limited commercialization, friendly people, and great dining and diving. Carambola is peaceful and isolated, a tropical paradise surrounded by a rain forest and cooled by the trade winds off the Caribbean Sea. All the rooms have a fabulous screened-in porch, which lets you be "outside" even in inclement weather. The beach is breath-taking and never crowded, even when the resort is full. If you are looking for nightlife or a resort for children, go elsewhere. If you are looking for peace and quiet in a romantic setting, Carambola is a great choice. The service can be spotty and the food expensive, so you would be well-advised to rent a car while there.
pal13, not that you are considering it, but we really loved St. Kitts. It is the complete opposite of Aruba: mountainous, green, a bit rainy on occasion (it never rains in Aruba), very undeveloped, and definitely not the cleanest, most prosperous of the Caribbean (which Aruba certainly is). But we still felt very safe there and it was unspoiled, uncrowded and very un-commercialized. Marriott is the only major hotelier on the island, and the resort there is very laid back and relaxing with an uncluttered secluded beach.