With only a week, I'd say do one, not both. For archaeology, see the INAH Museum of Anthropology, Templo Mayor downtown (along with other sights around the Zócalo), Teotihuacan outside of town, Xochicalco and Taxco for a day south visiting an important archaeological site and colonial city known for its lovely setting and silver, etc. Xochimilco and it's floating gardens (even a little known nursery and conservancy for the critically endangered axolotl), and an amazing amount of architecture, art, food, music, etc.

Another time, preferably Feb, Mar, Nov, visit Yucatán and visit Chichén Itzá, Palenque, Dzibilchaltún, Uxmal and the Puuc Hills and other archaeological sites, as well as lovely Mérida, the White City. Definitely worth a separate trip, but not when it's hotter than the hinges of Hades' gates on a busy day. Summer in Yucatán isn't even for mad dogs.

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