the difference in poly or nylon:
Nylon (polyamide) thread will melt and fuse at a low temperature. Nylon thread goes brittle, yellows over time, and melts at low temperature. Unfortunately, most invisible monofilament thread is nylon.
Polyester monofilament invisible thread does not go brittle, or yellow over time or melt at low temperature. Polyester embroidery threads are made of long stapled fibers with a silk like finish which are them durable and color fast. They will not shrink or stretch so your embroidery motifs will not distort over time.