I don't know about the resorts but can give info on islands.
St. Lucia is one of our favorite cruise stops. It is always very green and lush. The Pitons are amazing. The people there are very friendly, happy, and not pushing souvenirs. I would avoid the volcano--it's just smoldering ruins. It's one of those ports you hate to see the ship pull away from.
We have been to Aruba a few times and yet to find a clean beach we are impressed with. It does have casinos if you enjoy that. Like pp said it is like a desert--not lush like St. Lucia.
Finally, the DR is now what Cancun was 20 years ago--cheap, mass market beach trip. It does have some nice beaches, but people go there because of the price, not the beauty.
Your points are about the same regardless. I would go with St. Lucia. I'm sure you will love it.