i have never seen any plastic thread, but the poly invisible threads are a bugger to work with, they stretch, break, tend to be (picky) like those tags in your shirt that have a thread poking out bugging you to no end...same stuff, like heavy fishing line. and it melts and breaks with time. the nylon invisible threads are a little easier to use, i have found i really hate going that route! you have to really slow down to keep the thread from stretching/breaking. a medium gray usually blends in well (smoke for darker fabrics, light gray for lighter fabrics) if you want to use the invisible threads loosen your tension just a little, use a good thread in the bobbin and take it slow...the faster your machine is going the more stretch/breakage you are going to deal with.