Just recently purchased the fabric stamp from Able-labels and was trying it out last night :-
When stamped onto M&S white shirts (stamped onto double layer at the top/back of the shirt) it does show through but only if you look closely/press the double layer together, dont think it will show when wearing normally.
When stamped onto polo shirts (without double fabric layer) there is a shadow showing, but on ours there is a shadow from the garment label anyway. I expect name tapes would leave a shadow too. You could stamp onto the hem instead (but not sure anyone would look there if lost). Text is not as crisp and clear on polo shirt fabric as it is on shirts.
When stamping onto sweatshirt (red), had a bit of a problem with the stamp - will try best to explain. The stamp has a piece of double sided sticky tape that holds in place a piece of plastic personalised with your name, the sticky tape is bigger than the plastic name piece so there is lots of exposed sticky tape around the plastic name piece. Wasn't a problem when stamping smooth material, but, when stamping on the inside of a sweatshirt the pile on the sweatshirt stuck to the sticky tape and pulled it off a bit around the edges causing subsequent prints to have smudged bits around the sides from the raised sticky tape. Have pressed the sticky tape back down and seems okay now (exposed bits are losing sticky-ness). Again, stamping inside a sweatshirt is not as sharp/clear as on materials like shirts, our sweatshirts have a large labels inside so stamped on them too.
Haven't tried on trousers etc yet, but think they will be okay as the material is flat like shirts. Also havent tried washing yet to see if it stays on, especially on the M&S stain away material.
Also print is smaller than I expected from pictures on web site.
I went with the fabric stamp as I though the clothing marker would be very small letters and less clear.
After saying all that it took seconds to stamp, saving all the ironing and sewing, so if it washes okay I am pleased with it!!!!!!!!!!!