some machines cannot use the prewounds at all due to differences in the size of the bobbin case, so you need to check to see if you even can use one. As far as type of thread, you'll see folks on both sides of the fence, so you can do whichever you want. I use mainly cotton, but often use a poly blend if I see a color I want to use in it. I've heard that the cotton wound poly threads are very good because the poly core provides strength while the cotton outer layer provides the protection for cotton fabric. I rarely, if ever, use straight poly.