I posted this question in a video review about the Kettwiesel, and the owner wrote this:
"First, I have had a difficult time finding the variety of accessories for the Kett that I could easily find for my Greenspeed and ICE. I don't think it lends itself very well to being packed for touring.
Second, it is less stable than a tadpole. I never flipped a tadpole but didn't take me long to roll the Kett. Definitely my fault but it isn't as playful as a tadpole.
Third, if you are climbing a steep incline, say goodbye to a simple right turn. If the incline is really steep the drive system will cause the nose to drift to the left unless you plan way ahead to accommodate this tendency. Keep in mind that mine was a single wheel drive. This problem would not exist with dual wheel drive.
I would never consider the Kett for touring but I would take it over a tadpole any time for just plain fun in and around down. It can turn in its own length and weave traffic and obstacles in ways that a tadpole never will. It's a gentleman's (or person's) trike. It is far easier to mount and dismount and can pull a trailer or carry a ton of groceries. The Kett wins the fun factor for around town. The tadpole wins for touring. Hope that helps."
The very detailed video is here: