Ruby rides in a crate in the car. I like the fact that she is confined should there be an accident. Also on the crate vs. seatbelt choice, Ruby was already a well traveled dog in a crate (vari-kennel type was what her breeder used) before I got her. However, I prefer the crate anyway because if for some reason she wasn't feeling well, the cleanup would be a lot easier in the crate than on my seats (if she were belted).
It definitely wouldn't be safe to have her running around loose in my car (and trust me, that is exactly what would happen).