The difference is very big picture: LDS Christians believe that folks in other faiths do have the light of Christ & teach many good things (intermingled with not-so-good thing). I know many converts from Catholics, and they bring many good things: their love of Christ, priesthood, scripture, tradition, etc. But yes, they do need to bring some false things behind (such as the extreme glorification of Mary). But there is still much good there & we celebrate that.
Versus: The Catholic church doesn't even acknowledge the fact that I'm a Christian. What good does Catholicism see me bringing to the table? (these are honest question, I'm asking for the purposes of listening to the answer).