Arcade is indeed the original.
Neither port does the original justice, imo. They both share problems, and have their own unique problems.
The original is very tense, difficult action-platformer game. Even with giving yourself the option of early grinding, the game can easily overwhelm hasty or unprepared players with death - even those with the false security of killing pig-men for five minutes at the opening screen. That being said, the game was still designed for a methodical player to beat without grinding any experience points. The arcade original's four characters almost act as a set of difficulty levels, with the Priest being the easiest, due to her long range + invincibility spell, and the Mage being one of the hardest with his low HP and poor attack - mitigated somewhat by his incredibly useful offensive spells, at least on the enemies that are not immune to magic!
The giant level maps give the worlds a feeling of being interconnected, and I like the variety of the adventure - between rescuing a mermaid, or avenging a dog's dead owner ("Now you have avenged my people, woof woof!"), or slowly mapping your way through the final labyrinthine castle. All the while carefully managing your resources by stocking up on herbs, finding the two hidden elixirs, and spacing out trips to the inn (as the prices rapidly increase to beyond your means very quickly).
There are some things I am not a fan of in either port. For one, both remove the time limit and the increasing price of inn stays. This essentially removes the tension of knowing which paths to take (and what risks come with them) and completing levels as quickly and efficiently as possible. IMO, that's a large part of what makes the arcade game enjoyable. Further, removing the inn price increases means that the games are extremely easy because a full heal/MP restore is merely a trek back to the nearest inn. I like that the inn's in the arcade game give the foolish player the belief that they are a resource to be freely used, whereas a more experience player will push him or herself to the edge to make those early cheap visits to the inn count.
There are also minor things, like removing the secret artifacts in the crypt level (like the charmingly goofy little version of your sprite you carry around on your head), or making the mermaid scales an item you pick up, or having to necessarily compromise on the huge maps. They are less important, but they give the game its flavor.
The PCE version really should have been a CD port, as being limited to the HuCard just makes too many compromises: the graphics lose that nice grittiness by being too colorful and tiles too cartoonish, and the screen transitions are a little lame. While all four characters are there, the priest isn't as powerful as her arcade incarnation (flail doesn't seem as effective... just me?).
The MD port gets rid of the two most interesting classes, and its hit detection seems a bit worse than the PCE port and arcade.
While in some sense it's novel to see the attempt to port the game and have two different ways to play it in English (though lord knows how Working Designs "modified" the HuCard, if at all), unlike other Taito ports of that era (Darius II, New Zealand Story MD*, Kiki KaiKai), Cadash didn't make the transition very smoothly.
Now that I've talked up the arcade game so much, let me recommend you boot up MAME and play the US version of Cadash if you are interested. If that is too hard, the World version is easier (as it locks inn prices at 5000 gold) and lets you carry 8 herbs at a time instead of 4. The US version's translation can be a little stiff, but the World translation is hilariously unintelligible ("Thank you for the important thing!"). The World version is a good way to get acclimated to Cadash and go for the tougher JP/USA settings O:)
*the MD port has some issues, mainly the lack of warp points (that does hurt), though TNZS stands as an interesting console port of the prototype/location-test layout of the game, which makes it unique
