I have a commission due this Christmas, it has to be on a 40 x 30 canvas, and the whole drawing has to be in charcoal, so I want to know if it'd be the same thing as if I was drawing on paper. Or should I change to paper instead of a canvas? What would work best?
It depends on what you want to draw, and in what style.
Working with charcoal on canvas will really emphasize the texture of the canvas, creating a rough impression. It will be hard to draw details - in relation to the size, that is: you can still draw details but they'll have to be larger or implied rather than drawn carefully.
Coincidentally I've recently visited an Ilya Repin exhibition, and he made a few charcoal drawings on canvas as well (I'm not sure if they were preparatory sketches).
Below is a detail of approximately 40 x 71 cm (more than twice the size of your canvas) from one of those works:
Ilya Repin | Portrait of Vera Pushkareva (detail) | charcoal on canvas | 1899.
The complete work can be seen here. Mind that facial features and other details are drawn with pencil.
As you can see, a lot of the texture pushes through, and creates a noise, bringing to mind high ISO photography, or old television imagery, at times even glitches (where the canvas has a repeating pattern, which will be more obvious on linen).
For sketches it works quite well since it's easier to create an impression, as a higher amount of noise tends to be easier to be interpreted visually.
All this is naturally dependent on the roughness of the canvas, as you will be able to find canvases with a very fine weave, but never without texture (and will likely cost you more).
Another option, if you've set your mind to working on canvas but don't want to sacrifice detail, is to ground the canvas with an acrylic base and wipe it level with the bumps in the texture. It's not easy to get it perfectly smooth, though.
I assume a drawing on paper would be framed, so both objects would be around the same size, with fixed dimensions. Unless you're framing a drawing on canvas, it would be more receptive to dirt and damage. But it would also be more of a statement.
I've created numerous paintings with rough sketch on the first layer of canvas. I would imagine it would depend on the roughness of the canvas. I have not personally tried to do a final piece on a piece of canvas with just pencil or charcoal. Only light outlines. Prime canvas would be even more difficult to achieve because it gives a waxy layer on top of the canvas. The pencil would have a hard time sticking to the canvas without wiping it away. I don't think it is impossible. Like Joachim mentioned, it would be have grain and noise element to it as well, which would be a learning curve.
Pros: different, broad strokes not as much detail
Cons: Not for beginners, canvas texture learning curve