Yeah, the biggest difference is in the finish, as some others have noted.
Car or airplane, you want to have a clean build, so all of things you do to get a clean build-dry-fitting, cleaning up seams, preparing surfaces, etc-are pretty much the same.
But since it's most common to show an airplane-a military airplane, at least-with all of the wear and tear that service causes, it's a complete 180 from the traditional or most common way to finish a car, with a pristine showroom finish.
I had trouble going in the other direction, the first time I built a car model. I wanted to weather the heck out of it.
And before anyone says it, yes, junkyard finishes and dioramas are a style for cars, but not the most popular/most common.