Ted,
They can not take much of a beating. The hull is a ruberized fabric, and will rip if you run into a reef. Also, draging it up on a beach will gradually wear it out. They are not nearly as tough as a sit- on-top.
They are, however, more stable than a sit on top. At least my two person Folbot is.
I think local or travel is not the most important question. The most important question is if you want to camp. You can carry much more gear in a folding kayak than in a sit on top -- enough to sleep and eat comfortablly for a week. And a folding kayak makes a much better dive platform than a "normal" rigid sit-in-side kayak because of the open cockpit and wider profile. So I would recommend the folding kayak to anyone that wants to do multi-day trips.
Jon,
Yes cost is a big differance. Mine cost $2000. To me diving with the aid of an internal combustion engine (zodiac) is like diving with the aid of compressed air: the dive loses the aesthetic appeal. With kayak diving you are less dependant on technology and more on your own physical abilities.
Sven,
I folding kayak is delicate. You have to be mindful to not be washed up against a shallow reef. They do not, however rely on bulkheads to keep them open. The front 1/4 and back 1/4 are pure kayak in the eskimo sence: Covered for protection from inundation from waves. The middle 1/2 is open. Into the spaces in the bow and stern you typcially will insert a conical bag stuffed with food, clothes, tent, and cooking utensils. These conical bags are then inflated so they fit snuggly in the bow and stern compartments. A folding kayak does flex a noticable amount in 5 foot waves of short wavelength. This is of no conern, however and is actually desirable, as it makes for a softer ride.
So to sum up:
Typical, sit-in-side, rigid kayak:
Cheap
Lots of storage
tough
Typical, sit-on-top, rigid kayak:
Cheap
Easy to dive off of
tough
Traditional, sit-in-side, folding kayak:
Lots of storage
Easy to dive off of
Can take on an airplane