I personally like Isabelle because I used to feel like tortimer would keep those boondox(not sure if that's correct spelling) donations so he seemed untrustworthy. Plus Isabelle is cuter.
Tortimer was an elderly, half-asleep piece of lazy butt that no one ever really cared about when he did come out for things. I can't imagine he's doing that much on the island either. He's also a self-centered old fart. Heh heh heh horf.
Isabelle seems to be hard-working, on top of things, caring deeply for your town and your mayor, always wanting to help, and always nice to everyone.
I honestly think that Isabelle is much better. Tortimer came off as a cranky mayor that didn't give a crap about the town, at least that's how he came off on me. Isabelle is a spunky, peppy, ready-to-go secretary that loves to help people, especially the player.
I like Tortimer more. He has a more developed character as opposed to Isabelle who is just kind of generic. Yeah he's cranky and lazy, but characters like him and Phyllis make the game more interesting than just having all nice characters. It seems like Tortimer was created to be like a lot of politicians in real life.