As a former Vancouverite, I rarely stay in the hotels here, but I have experienced both of these properties. The location of the wall centre is nice being close to robson street and pacific centre mall but the bayshore is right next to stanley park and the water and is surrounded by the coal harbor area which has a lot of great restaurants and also is not too far from robson and other shopping/activity areas. It really depends on what you are in town for! Also just as a general notion between sheraton and westin. Westin's seem to have just that little extra attention to detail that makes ones stay great instead of just good! (Plus loving the new "Breathe Westin" program!)
Enjoy your stay in Vancouver, if you need any tips feel free to pm me!
Happy travels!