Aside from queue they're pretty much the same thing. I don't get why the name of the ride was changed from The Little Mermaid: Ariel's Undersea Adventure (DL) to Under the Sea: Journey of the Little Mermaid (MK) especially since the latter creates confusion with the Voyage of the Little Mermaid show at DHS. Either way most people call it The Little Mermaid as it is. I don't think any other ride that was cloned had this same issue did it? I wouldn't count the Finding Nemo rides because they are two completely different experiences that uses similar characters and moments.
Anyway sorry if this was brought up already. I'm rather surprised by the complexe and confused names, considering how oversimplified their movie titles are nowadays.