Cotton duck is a lot cheaper because it is made of shorter threads that have been woven together. The result is that it is weaker than linen (yet still sufficiently durable with the right treatment), and the grain looks different. Cotton tends to be lighter in colour. It is available off the roll primed and unprimed, in a range of grains and varying weights, we recommend a heavier weight canvas for larger paintings. The rest of the decision making process is down to you and your aesthetic preferences.
Art Supplies from Jackson's | Artist Cotton Canvas by the Metre & on the Roll: Extra Fine | Jackson's Art Supplies - Jackson's Art Supplies - Finest Art Materials - Best Prices https://www.jacksonsart.com/en-ca/surface/canvas/by-the-metre/cotton?surface_texture=14723