I haven't stayed at either, but have wandered by both. Belizean Shores has a beautiful pool, really looks to be a great place to hang out.
I greatly prefer being closer to town, able to walk if I want or take a short cab ride, so I'd go with Xanadu. Lots of people stay up north and don't seem to mind taking water taxis and such back and forth to town, but I'd rather be closer.
Both are great properties, just depends what part of the island you want to be on.