Agreed Le Mans has great footage and McQueen but there is almost no plot because there was almost no script. If you've seen the documentary about its making they shot great racing footage but changed directors during shooting because no one could find a plot in it anywhere and they were way over budget. (Like all things racing
) I believe McQueen even waived some or most of his payment to get the thing finished. McQueen wasn't a big dialogue guy and Le Mans is all about the cars.
Need to look at Grand Prix again. Don't remember it well enough but surely with Paul Newman, James Garner, and good international cast there is more service to a story line and plot. More conventional hollywood drama that isn't completely true to racing but was as close as you'd get for the time. I believe Grand Prix got both Newman and Garner hooked on racing.