The development of the QX was a mistake on Rossi's part. We all know that many thousands of QX sub units would be a nightmare to control and a waste of material to manufacture. He must have been greatly affected by the failure of the wafer based reactors and entered a state of inventors neurosis. So Rossi designed the smallest reactor that he could build without regard for power density, the most desirable feature of a reactor's design. Rossi must have been thinking about the home reactor as a QX application. But a home heating system still needs to produce about 100 kilowatts which is extremely unwieldy for the QX to meet.
I will bet that somebody other than Rossi came up with the SK design. The SK is recognition that the QX development was a waist of time. There must be an as yet undisclosed high flying reactor designer working incognito in Rossi's crew or in the employ of Rossi's customer.
The more power that the SK can generate makes for a better reactor system's design. The 20 kW E-Cat SK is a good system's design and I have my doubts that Rossi was disposed to think of producing it having been traumatized at Doral.