Hmmm, that's a tough one! I'm going to say that "scratching" is probably the worse of the two. Generally, a scratch on a car would be more extreme than a scrape. A scrape would probably involve leaving a scuff mark, but probably only temporary - but a scratch would actually remove paint, and leave a permanent mark. However, if someone says something like, "I need to go scrape my car [off]," and they live in an area that gets snow/ice in the winter, then they may just mean that they are removing the snow from their car. In that case, scrape would not involve any actual damage to the car.
Hope that helps, let me know if you need further clarification!
I agree that you can't really tell which is worse until you know the magnitude of the scrape or scratch. I would offer, however, that when someone comes through an accident or harrowing experience unscathed, the expression is "came through without a scratch." To me, this implies that the smallest injury would be a scratch.