Just got back on Sunday. We flew into Montreal on a Saturday. Stayed until Tuesday and left for Quebec City. Left Quebec City on Friday to head back to Montreal and fly out Saturday, but the Crowdstrike delay pushed our flight home back to Sunday.
We liked Quebec City way more than Montreal. If we went back we would probably skip Montreal all together, but if you haven't been it's probably worth spending a day or two there. Check out old Montreal, Molson Brewery, Notre Dame, Olympic Park, World Fair Park/F1 Track. We also walked around the farmers market in Little Italy.
Quebec City was awesome. Mostly just walking around and checking things out. We're Catholic, so walked through the Holy Doors. Saw the Citadel, went to the big hotel, and hit some bars around the area. One day we went out and saw Montmorency Falls and then went to Ile de Orleans. We drove around and did wineries and cideries, you can probably spend an entire day doing the waterfall and driving the island. Beautiful houses and scenery out there. The countryside on the island reminds me of the European countryside and Quebec City reminds me of a European city, lots of restaurants and stuff in the streets. Very beautiful city.
If I did it again I think we would fly into Montreal, do an afternoon and then a full day there and then head to Quebec City the next morning. Probably spend the afternoon and an additional two full days in Quebec City and leave the next day around lunch.
I guess in summary 2ish days in Montreal and 3-4 days in Quebec City is probably enough. Ate at some awesome restaurants in both places, happy to tell you some of those too.
ETA: I've heard Montreal airport can be a nightmare to leave from, but we got there at noon for our flight at 5:00pm and walked straight through security. Took us maybe 5 minutes to get through US Customs at the Montreal airport