First, a disclaimer: I've watched Grand Prix many, many times, including twice in its original Cinerama format when I was 13 years old, and the only version of Rush I have seen was on TV and the Spanish-language version at that.
The racing footage in Grand Prix was astounding on the big screen, and has not lost much in the way of impact over time, even in smaller formats and on the small screen. Cinerama had the distracting feature of the three-way split screen, but you could get used to it and it became unnoticed. At the time it was the best attempt to widen the field of view while sitting in a theater seat, and it was fairly successful. One would think if Grand Prix were produced today, it would be in IMAX format.
The racing scenes in Rush were good, and it was great to see the cars of that era, but I didn't think its racing scenes were as compelling as in Grand Prix. Again, small screen experience only.
As far as story lines, well, they are both movies, even though Rush did a pretty good job of telling the most well-known aspect of the 1976 GP season. I get it, Grand Prix had to get butts in seats, and date night generally means selling two tickets, and the off-track drama was not too bad, given it would last only until the next race. At least the drivers were not depicted as at each others' throats between races which would have been stereotypically Hollywood, and worldwide cinema production in general, for that matter. A big factor for me was the appearance of contemporary drivers who I had read about, and maybe seen giving answers to "What's it like out there?" questions, actually having "roles" (as such) in the story. It seemed to add an authenticity that is still apparent as I watch it today.
Rush was at a disadvantage in that it had to recreate a past rather than record a parallel timeline. Rush effectively presented the story of Lauda's unlikely, almost unbelieveably rapid recovery from life-threatening injury, which scriptwriters would have been challenged to come up with on their own. But even in Spanish I detected some "Ron Howard moments" that have spoiled some movies on which he had been doing so well, until...
BTW the voice actor who dubbed "James Hunt's" Spanish lines sounded very much like the voice-over guy in the Mentos commercials from the 80s.
In reference to the OP's tradition - for a few years in the mid-70s one of the local TV stations would run Grand Prix the evening after the Indy 500. It was shortened into a 2h30m time slot including commercials, but before VCRs, DVDs, or streaming, it was the only way to watch it when it was out of theater distribution. So I did. Now I have the DVD and I still watch it a few times a year, though I admit to FF'ing through the JP><Monique conflict and Jeff Jordan giving the scribblers a quick shmooze followed by Stoddard getting inside ASAP only to be scolded by Jordan re: trophies on display.