The Westin Rio Mar is beautiful, but a bit pricey. And you have to keep in mind that it is not in San Juan. If you want to spend the night in town, getting there is going to cost you. However, if you would rather stay within the complex (there's lots to do), it's a good choice.
If you need more information about this and other hotels, you should visit Puerto Rico Start, an informative (non commercial) list of links about the island.
Enjoy your holidays!