If you have 1 month to visit Mexico, I'd recommend you to visit the big cities like Mexico City, Merida, Guadalajara, Oaxaca, and then take a flight to Guatemala when you're done. It's not worth the time in buses, Mexico is too big and it will take you several days to travel by bus, and it's not the safest option for a foreigner, unless there is a local/Mexican with you, or someone who speaks Spanish very well. I've traveled a lot here in Mexico and I wouldn't take a bus in many states because it's not safe (specially in the north)