We have access to both here in Western PA. 99% of the funeral homes sell Wilbert concrete or some variant concrete. (We have many small companies which make concrete vaults of varying quality.)
I buried the last of my family before I started mort. school so I never knew about the Clark metal vaults until I attended PIMS OnLine. During that time we learned of the theory of them and I had a chance to have discussions with my on-line classmates from all over the country about their usage. Many of them are strong Clark supporters.
I've had a chance to see and examine one Clark vault and to be honest, I love them. I assume that you've studied the physics principles behind the inverted lid (diving bell) design that keeps water from rising, and keeps any condensate from touching the casket. There is some sound science behind the Clark design (and any others of similar nature).
From what I've seen in our area, the pricing of a basic Clark is not too much different from a concrete vault but the cemetery people don't like working with them because they have to be a bit more precise in setting them - so to stay on good terms with the grave diggers, the FD's just don't mention the Clarks or show them.
In the end, every body will decompose so much of the issue is psychological I guess. I think that a vault that has the Clark design does have the ability to keep the contents dry for a much longer period of time and this may be of some comfort to many people. (Of course, we're assuming that a proper and thorough embalming was completed or it all goes out the window anyway.)
I personally have come to the conclusion that for a given amount of money, one can purchase a less expensive casket and a Clark type vault and ultimately have more protection of the remains than if using a concrete vault or grave liner. They can also be a definite plus when selling a nice wood casket.
I personally would be showing and offering Clarks as an option all the time but the boss won't permit that. Had I known that they existed, I would have put my mom and dad in them but just didn't know.
I do have instructions to go into one myself, if that means anything and I know two other FDs who have plans to go into galvanized steel Clarks for themselves and their families.
Have you visited their website? It's pretty interesting.
Hope that helps give you another opinion to think about. In the end, it probably doesn't really matter but I personally felt cheated out of not having the option once I did learn of them. Again, it's much marketing and psychology but those can be just as important if not more so to the family than any science facts or fiction.