Stayed at the Westin Taipei last November. It's an Atrium type hotel, room was fine, we were rarely there. But comfortable enough. Service was good, geared mostly to the business traveller. Didn't really consider the Sheraton as it was still under renovation at the time. Depending on where you are going in Taipei, not sure which one is more convenient. The Westin is only a couple blocks from the MRT, and within walking distance to shopping/food (well, everywhere in Taipei is convenient to food), etc.