I voted NASCAR on "competitiveness" since more drivers have a chance to win any given race. The rules dictate more equivalence among teams.
However, I agree with Milo - F1 is essentially the pinnacle of racing. When you are talking about controlling a car that weighs about 1500 lbs that revs to 19,000 rpms (vs. 9000 for NASCAR) - you are talking about a real hand-ful!
F1 is popular because it is the cutting edge of auto racing technology.
If you want competitiveness, IROC is the way to go - identically equipped cars. NASCAR is closer to the IROC model than it is to the F1 model, hence more competitive. The IRL is the NASCAR version of open-wheel racing. So if you like competitive racing but want to see more speed - check out the IRL. As I said in another thread, the IRL cars circle the 1 and 1/2 mile at Chicagoland at speeds up to 230+, while NASCAR is about 185+ at the same track. Add to this the fact that bumping in IRL can launch your car into the fence at over 200 mph - it's pretty intense.