Just completed 2 stays at Sheraton (2 nites) and Westin (2 nites). The Sheraton Taipei is definitely nicer in terms of decor and overall maintenance of the room and hotel.

Both got upgraded to the suite.
Got L Occitane in the Sheraton executive suite, and Molton Brown at the Westin.

Sheraton is in a more convenient location for me, near to the Taipei Main Railway station, and subway is just 15 sec from the hotel. 10 - 20 mins by taxi to most location I visited such as Ximending, Taipei 101 and the nearby world trade centre, night market such as Shilin and HuaXi Street.

Westin service is also not up to par, esp at the lounge. Sheraton lounge service is much better. Jovi at Sheraton is helpful, while Joesph at Westin is probably the only employee that cares.