That's hard to answer. I have owned both.
Cats are smart - my Grandmother had one that remembered all the "bad" things I did to it as a child (liked to dress the cat up in doll clothes) - even after I was in my twenties and would visit. The cat would run and hide under a bed until I left the house.
However, dogs are smart in the sense they seem to tell you more of what they want when they want it. Plus dogs seems to sense/react more to human emotions and needs.
I have never heard of a service cat...so I guess dogs are smarter.