It really, really depends what you're looking for, and when you're going. I'm in Toronto all the time, and I lived in Montreal for 4 years, so I'm pretty familiar with both cities 😛
Toronto is big. Big parties, big events. Lots of crazy wild fun, in the typical mainstream sense. Of course, being a big city, you can find pretty much whatever you want, if you're willing to look.
Montreal is a little more off-kilter. The style and culture is a tad bit more alternative... lots of great music and great food, and tons of wicked little events in the summertime... giant street sales, that sort of thing. Not speaking french isn't a problem, but it can be an irritant.
But yeah, it really depends on what you're looking for and where you're going, like I said.