Curacao, I highly recommend a "best of" type tour, especially one that includes a visit to the Curacao distillery. The last time we visited Aruba, we went to Palm Beach. It gave us a little view of the island since it was across the island from the port. It was a very nice beach, with ammenities, and not very crowded. I think the cab fare was under $20, but it's been a while. The beach was free, you could rent chairs and umbrellas, I think it was $5-$8 for each. We did a great snorkel excursion in Grand Turk through the ship. There's a little shopping area where you port as well as a beach. LaRomana we did a great snorkel through the ship as well, with a nice beach day.
If you're a shopper, Curacao is the best place. Just take the little ferry across the inlet to the shopping center and go crazy. LaRomana was VERY expensive, they wanted $65 for an island shirt that at the most was worth $20. Aruba and Grand Turk are pretty flat islands, but very clean. Curacao is very colorful with a little bit of hills, but more lush. A lot of people took a tour over to the mainland from LaRomana, it was an all day adventure and I really didn't hear anything that would have made me want to spend my day that way. However, you might want to do that as there are some churchs and historic areas. We just had a great snorkel/catamaran tour in the morning and relaxed on the beach for a while. There are a lot of vendors there, but you can say no and they'll leave you alone.
Hope this helps a little.