It only matters a little bit where the car is assembled or what nation puts the parts together. The difference is where the money ends up when all is said and done. In this case even if the American car is 100% sourced and assembled in Mexico, the money flow still goes into the American tax system and into American cities.
That being said, I'll still always buy the best car that most suits my needs regardless of where it is built, assembled, or corporately owned. In my case it was an Evo X, but I did seriously look at American and German vehicles as well.