It's too bad you don't like snorkeling, Dominica is one of the best snorkels I've ever done. It's called Champagne Reef and you snorkel over vents where you can actually feel the water temperature change and if you catch the sun just right, it looks like you're in a glass of champagne. There is abundant sea life there, too. For hiking, Trafalgar Falls and the Emerald pool would be a good trip. St. Croix is not as built up as most of the tourist islands are. There are some beautiful beaches there.
However, Tortola and St. Maarten are great islands to visit as well. I'm not sure about hiking on either one, as we usually snorkel or go to the beach. We've done two great snorkels from Tortola over to near Norman's island and we did an island tour through one of the locals. St. Maarten we've done Orient Beach, Maho Beach, the beach near the docks, Marigot, a couple of snorkels-the last being at the Tiki Hut which was great, and the yacht races.
Sorry I can't comment on hiking, but I'd rather snorkel
Dominica certainly is lush and different elevations/terrain. St. Croix is more like St. John in the fact that it isn't as built up as St. Thomas. It does have elevation changes and lush areas.
It's a really tough call, but I'd probably opt for the extra day/island.