Agnieszka and I had only one day to explore Canyonlands NP and I would say that's precisely how much time you need there if you plan to visit only the main part (called Island in the Sky) of the park. However, if you would like to venture into the Needles District, you will need both more time and a four-wheel-drive (or be willing to backpack for a few days).
"Island in the Sky" is located just 30 miles away from Arches NP, so the two parks can be easily visited during the same trip. Compared with Arches, Canyonlands does not seem too impressive, at least on the surface. I am sure though that it is a much better place for backpacking than Arches.
Green River Overlook:
Grand View Point: