I used to paint, using either oils or acrylics, on stretched canvas, then when to canvas boards because they were cheaper and easier to store. Over the past year I have now gone over to stretched watercolour paper to save even more space and costs! I find that I don't really notice much difference in the painting experience, combined with the 'pound store' fine brushes I often use! I also get more detail in a smaller space on paper, still using the same oil/ acrylic paint.
I forgot to say for digital painting the costs of the 'canvas' and subsequent storage are so small that I do not think they can be measured in the real world!