That is the ultimate question. The Westbury is even better located from my point of view, but whether it's better in terms of the hotel I doubt. I don't think the new Anantara is ideal for sightseeing in terms of location, but it is new. Shelbourne is well located and pretty from the outside, but I'm not so fond of the photos of the rooms.
Overall, I can't say that the Merrion is not recommendable. It's just too expensive and the bathrooms are no longer dewy. On the other hand, the Shelbourne, for example, also had outrageous prices last time I looked.