I went to Aruba in April 2007 and did some scuba diving and snorkeling myself. For an amazing wreck dive where you will literally see thousands of amazing colorful fish the Antilla shipwreck is the way to go. I saw all kinds of angels, triggers, butterflyfish, snapper, grouper, wrasse, parrotfish, blennies, and more...I highly recommend it, it's amazing. The wreck is also encrusted with many colorful sponges, tunicates, and some coral. And oh ya, it's a 400 foot wreck...so you won't get bored!
As for reef diving, the best place on the whole island is Mangel Halto. It's right beside a beautiful lagoon, and is circled by shallow sand beds which have the most beautiful turquoise water. Beyond the sand beds, or a little walk/swim from the shore (depends where you go from) lies a beautiful pristine coral reef. Beyond the wave breaker is an even more pristine reef. The reefs have suffered little, if any, damage from humans, run-off, or commercialization. All the corals huge, happy, and healthy. Truly amazing. The water is a little choppy, but it's nothing that even an average or below average swimmer can't handle. You'll see gorgonians, huge colonies of different SPS, brains, sponges, soft corals, a variety of fish, and even sometimes a turtle, ray, or squid. It has an amazing biodiversity of life. Ask the locals for the exact location/information because it's not one of those "tourist dive spots", it's truly a beautiful hidden gem.
Also, both sites can easily be snorkeled and you still will have an equally amazing experience.