you're more likely to get those with a prime lens. a prime has less lens elements in it which usually means that you get better color and contrast in the image.
the 1.8 aperature will give you more dramatic changes in depth of field, however, it really depends what style you want.
for immidiate satisfaction, shoot the 50mm. maybe at a 2.8 or 3.2 just to get the car in focus, but get the background out, sample around with different aperature settings to see what you like, toss it on the computer, then see what works, then go back out and shoot it again, but keeping in mind what adjustments and things you liked, and apply it to more photos.