There are many kinds of crabs, including a few land dwelling species. On the west US coast, the most sought after are usually either the Dungeness crab or the Alaskan king crab. In many respects, they are similar. In other ways, they are vastly different.
Both of these crabs prefer cool water. The king crab lives in very frigid water, however the Dungeness also lives in the quite cold waters off the coasts of California, Oregon, Washington, Canada, and Alaska. Both crabs are also scavenger feeders. They will eat live food when it is available, however they will also eat almost any flesh that is decaying.
Both also scuttle about on the bottom of the ocean, and these habits are quite similar. However, it is about at this point that the similarities stop, except that they both look like crabs.
The most noticeable difference is the size. Alaskan king crabs are commonly more than six feet from claw tip to claw tip. The body is also slightly larger than with a Dungeness. However, compared to the body, a king crab has much longer legs. A Dungeness, which commonly measures less than a foot across the shell or carapace, seldom has legs that would double that size, from claw tip to claw tip.
A king crab, on the other hand, has a small body for the spread of its legs. Though it is a crab, it is similar to a 'daddy long legs', in having a small body and extremely long legs. This makes a huge difference when the crab is processed and canned. Most of the crabmeat from a Dungeness comes from its body, while most of that for the king crab comes from its legs.
It is no mistake that this is why king crab legs are often sold, while with Dungeness crabs, usually the whole crab is sold. This affords the person, in the latter case, to get the succulent flesh from the body of the crab.
An Alaskan king crab also more often feeds at deeper depths than does the Dungeness. Dungeness crabs at times work to deeper depths, but they are often caught off jetties and piers. The same can rarely be said of the king crab, though it does happen on occasion.
King crabs tend to live and feed in deeper water where the often-stormy seas don't have much of an impact on them. Dungeness tend to live closer to the low tide mark, though they can live in deeper water, because severe storms are not as common.
In flavor, there isn't a great deal of difference, though many people prefer the taste of Dungeness to that of king crab. This amounts to a personal preference, rather than one being actually better than the other.
The difference between Dungeness and king crams is most easily seen in the difference in size. However, there are also other differences as well. Ultimately, it falls on the individual to decide which they prefer. It can be a tough decision, because both taste great!