My parents have given up four of their six bicycles, due to age. Thats why I now also own a Brompton. Side by side comparison:
So I had time this last week to test Brompton more extensively than before.
I dont like the anatomical geometry of the Brompton that much, my back is bent like that of an angry cat, it feels like sitting on a racing bike, almost, and it causes pain in the lumbar region since the muscles are constantly overstretched there. I drive more relaxed and with much less effort and power on the Kwiggle - while being slightly faster than on the Brompton at highest speed I could go. Both bikes can go surprisingly fast.
On the Brompton, the driving is more forgiving regarding unevenness in the flooring, holes, stones, sticks. On the Kwiggle you have to be on your guard and scan the track ahead. Failing to do so can lead to desaster. Assuming you have smooth track surface and good road conditions, Kwiggling is joy, however, pure joy. Cobblestone is not.
On both bikes I feel safe and not in danger to fall, not during accelerating, not during breaking, not during turning. But the Kwiggle driving must be learned, you cannot lend this bike to somebody and this person than just uses it immediately like any other bike, it does not work this way.
Also, the pack bag Brompton offers is much smaller than the standard size bicycle bags I could mount on the front of the Kwiggle. Really, believe it. The Kwiggle mounts more luggage
volume than the Brompton with the Brompton standard bag for the back mount. However, the limiting factor is the
weight, the Brompton can mount luggage of more weight. Kwiggle is limited to 100kg: bike+driver+luggage<= 100kg. Its not due to the frame, that has a limit beyond 250kg, but its due to the air pressure in the tires, these are the critical factor, not the frame.
Its nice to have another backup, and for free ("dem geschenkten Gaul schaut man nicht ins Maul..."), but I prefer the Kwiggle, the driving must be newly learned, but then is superior.