I like Tortimer better. Though he seems to be lazier than Isabelle (in Animal Crossing games other than New Leaf), he's not as clumsy as she is. However, Isabelle is better at planning: ordinances, public work projects, and giving you input on the status of your town and how you can improve it. In some ways, Isabelle is like Pelly in City Folk.
This is indeed a tough decision, but I'm still going with Tortimer. His tours are fun lol.