I'd say it's worthwhile to split the time between Ottawa and Montréal: spend 2 days in one and 3 in the other. Exactly how you divide the time between them will depend on what you're most interested in: Montréal has a picturesque old town, a good historic museum (Pointe-a-Calliere), a good small zoo (the Biodome) and large botanical garden at Olympic Park, the Parc du Mont-Royale for hiking and views, and good dining and nightlife. Ottawa has the Parliament buildings, several excellent Museums (Canadian History Museum, Canadian War Museum, National Gallery), and a lot of good cycling trails along the rivers and canals, as well as Gatineau Park (a great place for hiking, if you have a car to access it). You do need at least 2 days to see the main sights in Ottawa. Montréal is, in my view, more a place you go for ambience, nightlife, food, festivals, etc. than for specific sightseeing; the old town and Mont Royale are the only real 'must sees'.