Specta is Polyethylene, just linked differently. Because of this, it stretches less. Specta is the same plastic as in the shopping bags. A Polyethylene rope that has been fully stretched in the sun for a few days will have similar stretch characteristics as the Spectra. It takes a while to get if fully stretched, but it van be done. The best example of this kind of rope is the Fly High Tak lines. It is light, does not knot or tangle easily, and works well once brokend in. A lot of other Poly-E lines will not fully set no matter how stretched due to the weave of the rope.
Spectra is a brand name of a particular kind of polyethylene that has a different molecular arrangement (linking) than standard Poly-e. It too has to be pre-stretched, but has less creep than poly-e or Tak, making it easier to use out of the box.