Synicsel

It's not unusual to prefer one over the other for specific projects. If you're looking for a stiffer #138 thread, you might want to check if there's a Bonded Polyester variant that suits your needs. If you're ever interested in exploring something else, I recently came across some interesting info about Polyethylene Terephthalate (PET). It's all about plastics and recycling, which is quite a hot topic nowadays.

The stiffness and softness you're experiencing can be due to their individual properties. Bonded Nylon tends to be softer and more pliable, which can make it prone to fraying. Bonded Polyester, on the other hand, is usually stiffer and more durable.