There's quite a difference; for one, Top Gear is VERY critical on American cars. Next, the Mustang is more of a series of models, as it has been around forever. They didn't like the new one, but they sure do like previous generations. The 350z is of its own; they don't like it as a whole, including its heritage.
Also, I don't think you realize this, but Hammond has owned quite a few American cars, including a "Mustang":
http://www.youtube.com/watch?v=i0s4OAMISN4&feature=related