heh, demo...
Playstation Magazine (US) had a nice comparison:
Grid: 41 cars, 14 locations with 72 track variations, and 51 events totaling 136 races (plus Le Mans).
GT5: Prolouge: 71 cars, 6 locations with 12 track variations, and 40 events totaling 40 races.
I can testify that those "variations" on Grid aren't very different, either.
Anyway, Grid is $60 and GT5: Pro is $40.