if you appreciate dancing and brilliant choreography and people singing while they are upside down and jumping around (not easy!) go see cats. That'd be my choice.
CATS is a dancer's show. If you are really into dancing and great choreography, definitely see CATS. Also, like someone said, if you are a cat lover, and you have cats as pets, you will probably enjoy the "felinity." It will make you laugh.
Joseph is a lot of fun - upbeat music, more of a comedy, tells a more followable story, cool lighting. Very campy but great fun. Amy Adams is quite good as The Narrator.
Of the two, I think CATS is more unique and demands more of the cast. Again, the dancing is unmatched (I like watching dancing, so I like it). But if you've never seen Joseph, and you like the music, it's a good bet. Joseph has dancing too, but it's not on the level of CATS.