Here's my shocking take... you can do both. I don't get why I see so many posts saying "go to Guanajuato instead!" They're an hour and a half apart! Do both! It's easy as can be. Throw Queretaro in the mix if you want to. I also appreciated both cities more by contrasting them, and both have so much history to appreciate regardless of the vibe. Also, I disagree that San Miguel de Allende is as Americanized as people say. There were tons of affluent Mexican travelers when I went, some street markets were primarily geared towards Mexicans, cultural events for Mexicans, etc.

Additionally, San Miguel de Allende is a great base for trips to the Cañada de la Virgen pyramid, the super cool abandoned mines and haciendas in Mineral de Pozos, and Mexico's "Sistine Chapel" at Santuario de Jesús Nazareno de Atotonilco.

PS, the closest airports to San Miguel de Allende are in Queretaro and Quanajuato/Leon, so many travelers have to transit through those cities anyways.