It sounds like a great trip!
We took some Australian travel mates on a similar (lengthier) trip - renting cars in Oaxaca, Villahermosa, Mérida. I can't remember what vehicles we used, but everything was fine - the most frustrating part was having to return to MEX to go from OAX to VSA and from VSA to MID. With new airlines in Mexico beginning to stitch point-to-point networks, maybe this will continue to become easier.
Thank you both. I think I'll rent a car and drive. I did this before and it was ok. Now, their road network is even better!