I’d say that if you have to pick just one to visit, go with Montreal. Toronto’s a great city but it’s essentially just another big city - it’s like visiting New York or Chicago or Philadelphia. Montreal has a unique character you won’t find in any other major North American city.
Just so there’s no misunderstanding, that’s a twelve to fourteen hour drive.
You aren’t leaving yourself a lot of time.
I grew up around Toronto and I currently live in Montreal and I love both. If you’ve never seen either then I’d say you’d be happy with whichever you choose.
Why don’t you pick the easier trip driving-wise?
Also, while I agree Montreal is unique because of the language, I have to back the few people up thread who said that Montreal isn’t any more hip than Toronto. Often I think it’s just thought of as more hip because it’s very important for the people here that they’re thought of as hip.
There is one upside to Montreal though. Because of differences in Quebec liquor laws, there are plenty of restaurants here where you can bring your own wine with no corking fee. It’s awesome and I’m going to really miss it when I move back to Ontario in the fall.
Exactly. Montreal has French. Toronto has English, French, Chinese, Korean, Urdu, Hindi, Spanish, German, Italian, Portuguese, Ukranian, Polish…
Photography? Skip Montreal and Toronto. Drive the north shore of Superior.
I’m from Montreal, so it’s my duty to tell you Toronto is lame. 
Actually, I haven’t been to TO since I was ten, so I can’t say much about it. But I love Montreal with all my heart and can’t wait to get back home for a visit.
Lake Superior Provincial Park north of Sault Ste. Marie is stunning. Go there. It’s about an hour north of the city along Highway 17. You won’t regret it. 
The best thing about Montréal and Toronto is that they are only five hours apart! You can take in both of them!
Montréal has: French. Stylish women. The Metro. The Mountain. Old Montréal. The Olympic Stadium complex. The Biodome. Restaurants. Galleries. Museums. Festivals. I like Montréal a lot and am going back in a month.
Toronto has: More languages than you can shake a stick at. Scattered examples of interesting architecture. The CN Tower. Restaurants, including Tibetan, Peruvian, and Kashubian. Galleries. Museums. Festivals. Little India. Koreatown. 5 Chinatowns. The Polish area. The Russian area. The Punjabi area. The Arabic area. The Distillery District.
[sub]I will admit that the Montréal Metro has better architecture than the Toronto subway, though.[/sub]
One thing I have heard from multiple sources is that Torontonians are much more ‘reserved’ than Montrealers. We don’t smile much, and it’s hard to meet people here, even if you live here. Montrealers are apparently more outgoing. If so, that’s a point in favour of Montréal.
Just to totally mess you up, have you considered stopping for a day in Algonquin Park? If the focus of your trip is photography, it might be interesting to see the basis of Tom Thomson and the Group of Seven’s inspiration…
The city that won the most Stanley Cups or the city that has the Hockey Hall of Fame?
Hmmmm…tough choice.