While I have not stayed at the Sheraton, between the two... I would choose the Westin Harbor Castle.
The Sheraton as the fine Mr. Lion says looks tired and old. Plus everytime I was there there were a bunch of "more than tipsy" people staggering around the lobby (there is a big sports bar in the lobby.) The staff is also not that friendly. Which brings up the question: Does the Sheraton promise cover properties in Canada? If yes i'm tempted to stay here and see what happens.
I have been staying at the Westin Harbor Castle for over seven years. It's tradition for me to stay here every fall when I go see a programme called "The Red Green Show." (See OMNI for threads on this very funny comedy show.) I've always gotten a harbor view room (Request it in my reservation or find it on the SPG.com website.) Staff always very helpful and you can get to downtown pretty easily with a short walk to the streetcar to the very clean Toronot subway.
The hotel did not have the Heavenly beds when I stayed there last October. I said "whatever, I am staying here for the staff not the beds." The bed I slept in was comfortable enough.
If they are there when I come back this year fine. If not then I can sleep on just about anything.
I have not stayed at the other Westin but I may try it sometime.