Gay Ann asked me to write about canvas and linen and the differences between stitching each.
There are two disclaimers I must make first.
This is the most important.
I am an embroidery floozy. If a new technique or a new approach to an old technique sidles up to me, I cuddle right up and bat my eyes, allowing it to whisper sweet nothings in my ear and encouraging it to make improper advances. A threaded needle is all it takes. I'll try almost anything that involves one. I tend to do mostly things that use tapestry needles since I am a klutz and I bleed easily, but I am never going to limit myself to tapestry needles.
Secondly, anything I say is my opinion only. I take a lot of classes and I stitch things from a lot of different people, but what I'm going to say is based on my experience and mine alone. Your mileage may vary. This is what works for me.
The most obvious difference to me is the ground fabric. Canvas (and I'm lumping Congress cloth in with canvas) tends to have more body, probably due to the sizing added to it. In other words, it's stiffer. Linen has a lovely, fluid hand--at least the really good stuff does. Canvas tends to have more consistent thread sizes in both warp and weft, while the threads in linen have much more variance. There are some really good linens which don't have quite as much, but you will still find a thread that is thin as a whisper while the thread next to it may look like a tree trunk in comparison.
These differences tend to make you stitch on them differently. Because of the uniformity of threads and the sizing, you can easily work over one intersection on canvas. You can work over one intersection on linen, but the stitch is a little more likely to slip between the threads of the linen. You also may want to compensate for the differences in linen thread diameters. To overcome this, most people tend to stitch over two (or even three) threads--this helps hold the stitch on the surface of the linen and allows the differences between the sizes of the threads to average out a bit.
You can do most of the same stitches on both grounds, but some of the really heavy stitches--like maybe a big Rhodes stitch--may be happier on canvas because the body of the fabric will hold it better. Some people say that cutwork and drawn thread works better on linen because of the fluidity of the fabric--don't know that I agree with that, but it is something to consider.
The one thing that I always do with both linen and canvas is mounting it tautly in some kind of frame. Canvas pretty much has stretcher bars. Linen can be mounted on stretcher bars, scroll frames, embroidery hoops, Q-snaps, and slate frames. Actually, come to think of it, canvas could be mounted on slate frames, too. I know people who do all their stitching in hand and do it quite well, but I find my stitches just simply look better to me if I keep the ground fabric, whatever it is, as tight as possible. Plus, if it's mounted and the frame is held by whichever frame stand fits the frame, I have two hands to use. I can stitch with one hand above and the other below, I can use my pinky for an emergency laying tool, I can hold the needle while I fiddle with the stitch with the other hand, I can control the tension of the stitch, I can do all sorts of things if I'm framed up.
There is one other difference I just thought of. The charting for the two fabrics is usually different. Both use graph paper. For counted canvas, usually the lines of the graph paper stand for actual threads and the stitches are charted accordingly. What you see on the chart is what your thread should look like on the canvas. On counted thread charts, frequently a symbol indicating color is found in the block the grid forms, particularly if the design is primarily cross stitch. If your design uses something other than cross stitches, you may have two types of chart. One has the color key, then you may have another chart showing how the stitches work using the lines of the graph paper as charts for canvas do.
Can you jump from one to the other? I do it all the time and can't think why anyone would want to limit themselves to just one. And then there's goldwork, and crewel, and needlelace, and silk and metal thread embroidery, and stumpwork . . .I really do need to live to be 385.