i'm going to chime in here because I have used all of these items...the pounce, the washable markers and the spray. I like them all, but don't use the same thing all the time. I don't think any of them are appropriate if you want to mark an entire quilt top and have it stay marked until you are done. For that reason, I sandwich my quilt and mark sections as I go.
I'm going to attach a pounce marked placemat. I think you can see the markings beautifully with white chalk on a darker print. On some of the lighter patches, i went in and filled in with a blue wash out so i could machine quilt easily.
I am doing FM quilting on a DSM. The trick to using the pounce is to really get that cloth filled with chalk. So I like to leave the cover on (and I even put it in a plastic bag to keep it from puffing out) and I bag it several times on the counter to get that chalk flowing. Then all you need to do is to wipe it back and forth over your stencil and it is marked. it does work. You have to experiment a bit with it until you get the hang of it. Read the directions carefully.
I personally love water soluble pens for marking and when I was hand quilting, I used that regularly. I like the collins brand. Make sure to use the water soluble and not air soluble or your lines may disappear. if you live in a very humid climate, the lines can disappear too. Don't leave marked items near heat, sun, irons, etc, as all of the above can set the markings. And only rinse with cold water, not soapy water.
These pens show up best on lighter fabrics, whereas the chalk (at least in white) shows up best on darker fabrics. it takes a bit longer to use a stencil and mark every opening with a washout, but you can fill in the gaps and make your markings quite precise.
And if you are marking without a stencil with a free hand design, like feathers filling in a spot or even stippling, this is great.
Lastly is the spray. Very quick, great markings. comes in gray or white. Disadvantages are a bit of a smell when you spray it (almost like spray paint), it does spray your stencil so you have to wash the stencil off after each spray session, and it does create a chalk dust for me in my machine, which means i am dusting out my bobbin case fairly regularly when I use this. But it is fast and the markings are very clear.
As far as cost, the spray is probably the most costly, but if i am in a hurry, I am willing to pay for convenience. (and I am always in a hurry!) Pounce pads seem to last a good while, so probably with a large bag of chalk refill, very economical. Pens are not very expensive but they do dry out, and you have to buy fresh ones pretty regularly. So don't stock up with 10 if they are on sale. Buy a few and then a few more.
There are other chalk pencils out there for marking which are also good. Chalkoline pen style. Very nice.
With all chalk products, if you are trying to mark an entire quilt and then start in on the quilting, I think you will find they will rub off and you are better off marking in sections as you are ready to quilt, IMO.