Coming to Montreal the middle of May then on to Quebec, do you suggest we rent a car and how is the parking?
you don't really need a car. while parking is ample (and uniformed ease of use) Montreal is amazingly walkable (especially mid May), most thins you will do in Montreal are in close proximity, and the taxis are fairly cheap when you need one. But there is also the Bixi bike system, it costs $5 a day, so you can pick up a bike pretty much everywhere you go.... no hassles no locks.... the drive to Quebec City is a nice drive and fairly easy. You can certainly rent a vehicle for that drive. Although again, the Train is hassle free, downtown to downtown... and if you don't normally read French highway sins, a lot easier. if you o to viarail.ca you can even find some great seat sales.... I would suggest you go car free
You can get around Montreal by Metro or city bus and avoid the parking issue. The drive to Quebec city is boring and the city is not so big so you don't really need a car when you're there. You can take a bus or train from Montreal to Quebec city (both are easy). If you're going further East of Quebec city or you want to linger North or East of Montreal in the "mountains" then you might want your own wheels (even for a day). Enjoy your visit.