If I can make a recommendation...
If you are there to see the drivers and cars up close and personal, GO FOR THE QUALIFYING DAY. I went to both last year. Qualifying day was essentially a ghost town, I was able to get into pits, talk to drivers and crew, pretty much all access. Competition day was a MAD HOUSE. I got there a little later than I had wanted to and it was packed to the gills. Its still all access and a fun time, just not as intimate.
Example of day 1 photo:
Example of day 2 photo, from the bleachers, with my 250mm lens at full zoom:
