I agree with all the others, do one or the other. You can't do both justice in one trip.
I've spent a lot of time in Montreal, but somehow never got out much. Mostly I spent time with the ex's parents.
Great things about Montreal:
The food: gotta visit one of the famous Montreal deli's. Mmmm, Montreal smoked meat.
Festivals: I've never managed to attend one, but the comedy festival is one of the best in North America, as is their Jazz festival.
Check here for a list of festivals. There is something going on pretty much all the time.
The food: this is a city full of incredible restaurants. Things don't get humming until after 9:00pm for dinner.
The Mount Royal cemetery. No, really. It's incredibly beautiful.
The food: Quebec never did kill off its small dairies. There are no end of small local cheeses to try.
The night life. Can't help you with the scene as I was last there more than 10 years ago.
Old Montreal. Lots of good restaurants, lots of history, lots of buskers.
On Niagra, the falls and surrounding parks are great. Give the rest of the town a pass. A winery tour is a must if you enjoy wine at all. I can make some recommendations if you do that.
There is also 40 Creek up near Hamilton (1 hour from Niagra towards Toronto) if you are into Whiskey.
Toronto's China town is always fun and the Art Gallery of Ontario and the Royal Ontario Museum are worth checking out. They both attract top notch exhibits. If you've been to the Boston Aquarium you may be disappointed by the Ripley's aquarium, but the main tank is cool.
Hmm, if you are in the Niagra area during the weekend, I'd love to meet up with you. I've always wanted to meet someone that
can put up with hangs out with Bob on a regular basis.
<- for Bob...
