The Kohs is an exquisite museum quality model - the Sunset a nicely detailed good running layout quality model. Side by side close up the differences would be very noticeable. Running 3 feet away I may not notice the differences as the overall appearance of a beautiful model machine in action is what gabs me. Each to his own value preferences.
I would like to see both Kohs and Sunset provide bi-modal DC/DCC electronics/sound in their steam locomotives like the HO importers provide. If you were to ask me which of the two models I'd like to handle and maintain I'd choose Sunset. I have a Kohs K4s along with a full page of notes on how to disassemble the chassis from the boiler to install DCC. It takes a surgeon's hand to connect and disconnect the valve gear actuator to a second motor within the closed bottom of the boiler.
Having seen the beautiful running qualities of N&W Chris' several Kohs Y6b's I'd bet the Challenger isn't a shelf queen. Perhaps some day we'll see a video of a Kohs Challenger in action.
Ed Rappe