To be fair, the Agfa Isolette III is a fine camera. Its only design problem is the original bellows, which can be fixed. Admittedly, the Perkeo is more handsome, the overall quality of the finish much more beautiful. It is even a bit smaller and the body has a more sturdy feel to it than the Agfa, but I would say that where it counts most - the rigidness of the folding mechanism - the Agfa is a little bit ahead of the Perkeo. Also the film transport/frame counting mechanism of the Perkeo II is known to be flimsy and indeed it broke down on my example after half a dozen of rolls.
I also have to remark that the Solinar of my Isolette III is quite sharp, indeed the slides I take with my Perkeo II/Color Skopar appear a bit soft and lacking in contrast in comparison. In the end, this might of course be due to individual sample variation in the lenses (infinity is adjusted correctly on both cameras), so your mileage may vary. So, to sum it up, when I travel and need a compact, reliable and sharp camera for 6x6 slides, the Agfa is the camera I take along while the Perkeo has to stay at home.
Whatever camera you buy, I would recommend to look for a folder with a Compur Rapid or Synchro Compur shutter, these usually have a full range of shutter speeds from 1 s up to 1/500 s. IMO these are very good and reliable shutters, and a lot of camera technicians still know how to service them.
The second variety of shutters that are usually found on West German post WWII folders are Prontor/Pronto shutters, and I must admit I have a personal adversion against them. The varieties that are found on the MF folders usually end at 1/300 s or 1/250 s (in reality this translates to 1/150 s), which leaves you with only one single usable shutter speed if you really want sharp results, as I find the next available shutter speed (1/100 s = in reality 1/60 s) already to be at or past the limit for handheld shooting.