I speak from a wholesalers perspective. The K2 rotors water your turf about as well as concrete brick. The heads are most often failing for one thing or another, and not to mention their water dispersion. Most people have believed the rumor that the same engineer designed the K2 / the Pro Plus and the Hunter PGP. But in fact, the only similarity is the tripping mechanism. It is the same in all heads (K-rain Pro-Plus and Hunter PGP). It was designed by a Toro engineer and it was adopted by Hunter in their PGP (if memory serves me correctly, so don't quote me).
If I had a choice, I would choose the PGP over any K-Rain products. Hunter is the number one seller of rotors for about 16 years now (Rain-Bird is a close second). As far as the PGP versus the I20, well there are some differences, but not enough to warrant a major decision. The main difference is the water shut-off feature, and another is the top of the head has a larger cap and rubber cover. I also believe there is a higher pop-up height (not real sure about this one). As far as the transmission / drive motor, if I am not mistaken, it is the same as the PGP.
Best of luck with your decision. <><
Josh
Irrigation /Landscape Lighting / Pump and Well Specialist