The oval shape is certainly similar. And both have extremely wide seats. Although nothing can compare to the Buggster's astonishing 18"; certainly big enough for me to ride around in! ;) They are both umbrella strollers with fairly deep reclines, and huge canopies. And both have a very modern look. But the comparison ends here.
You can start to see how different they look from the back. The Kooper has ergo, foam covered handles that turn in, while the Buggster's are straight up with hard plastic finger grips. The Kooper has two cupholders, and a zippered canopy pocket. It has a good sized basket, though not as big as the Buggster's. The Kooper has the new red brakes. Some people don't care for them aesthetically, but countries such as Australia now require them for safety. The fold on the Kooper is very straightforward, and on the Buggster it is much more fiddly. Although there are several versions of the fold on the Buggster, and some are easier than others.
The Buggster comes loaded with accessories, including a mesh seat pad, front napper bar, boot, sun cover, and rain cover. The Kooper has the aforementioned built-in canopy pockets, side net pockets in the seat, and glow in the dark shoulder harnesses and logo.
Here you can see the difference in the width. The Buggster is nearly 4" wider. But the length is where they really differ, due to the 3 wheels of the Buggster vs. the 4 wheels of the Kooper. 3 wheelers always have to be longer for stability, and in the Buggster's case, it is nearly 9" longer!
Both have a roomy 24" canopy clearance, although the Buggster's seatback is 2" taller. The Kooper folds much smaller than the Buggster, but longer. See the chart below for even more measurements.
Joovy Kooper Valco Buggster
Weight: 17. 5 pds. 18.5 pds.
Width: 21" 23 1/4"
Length: 26.5" 34"
Height: 40 1/4" 40.5"
Seat Width: 15" 18"
Seat Depth: 8" 9"
Seat Height: 18" 20"
Seat to Canopy: 24" 24"
Drop to Footrest: 11" 13"
Seat to Floor: 16" 16"
Folded: 46" x 13" x 12" 31" x 22 x 25"