I can't say exactly whom of them I liked better, but with his sometimes a bit unsensitive sense of humor Mo definitely had more of a distinct characteristic feature than Chomper. In LBT 2 Chomper was actually little more than a Dinah and Dana character. Being unable to speak doesn't disqualify a character from being important (Spike), but actually only the fact that he is a sharptooth made Chomper a somewhat more special case. The only characteristic feature about Chomper is that he is very sensitive in both movies where he appeared. I'd actually like to learn a little more about him.
One thing about characters who appear in one (or in Chomper's case two) movie(s) is that it is almost always Littlefoot who makes friend with the character or at least deals with the character more than the others (Chomper, Ali, Doc, Mo... even Mr. Thicknose whom Littlefoot didn't like and vice versa was mainly connected to him). The only exceptions were Dinah and Dana who by nature were more attached to Cera (but here again they are characters who can't be compared to Ali, Doc, Mo or Mr Thicknose), and Pterano (the only case of a really interesting character attached more to somebody else than Littlefoot).