Willis Ave. is a good spot because you have some options if you're taking pictures, and because if something good shows up, like say a heritage unit, it is possible to jump on 690 and beat him across town. I used to do that when TV 556 was coming through with SP units, although that train did work in Dewitt too. If you're up on the bridge shooting you also have the advantage of being able to see across the lake and see if any trains come in on the line going north.
Also, if Finger Lakes is running you can go see them. I'm not sure what NYS&W does these days but you may be able to catch them as well.
The only thing you might run into this weekend is people parking in that area for the Fair, you'd be surprised how far some folks will walk to save five bucks.
The mall is okay in the morning, will be packed and busy in the afternoon.