My husband and I have stayed at the Palms many times and it is our favorite place. We looked at Oasis when we were there in Dec. The condos themselves are very nice and comparable in inside accommodations to the Palms. In fact, it is a newer property and the condos we saw were a little nicer furnished than some of the Palms condos. Oasis is located a little north of town but certainly within walking distance to everything you would want. They have a nice pool on their small property that overlooks the beach.
Here's my viewpoint of the differences and why the Palms is more expensive: The Palms is on a much nicer beach, has better verandas and better views from the rooms. The Palms has on-site staff that makes the property seem more like a hotel than a condo. The Palms is next door to Ramon's (nice for the diving and the restaurant access) and a little closer walk to town. So basically you are paying extra for location, location, location.
Hope this helps.