When Nigel was in IndyCar he was just at the top of his performance, winning the Formula 1 World Championship a year before. If you compare that to others who moved from Formula 1 to IndyCars (including ChampCars) this was different. In the time Rubens could have been at the top of his performance I fear he was the clear number two in Ferrari. It is very difficult to judge though, since he raced against rookies in the Williams years. When he was in the BrawnGP however, he was not really on one level with Jenson. And, of course, Nigel was in a Newman/Haas, who cashed in the IndyCar driver's championship already in 1991.
I also think the current IndyCar has a pretty high density of talent, which can be seen by the number of drivers who had the potential to win a race when Genassi or Penske had their problems. Also the talent level in IndyCar is easier to be compared within the series than this is the case in Formula 1. The clear structures of how well a team performs vanished with the late drastic rule changes. Now you have different drivers in different teams on essencially the same speeds.
I also think the current IndyCar has a pretty high density of talent, which can be seen by the number of drivers who had the potential to win a race when Genassi or Penske had their problems. Also the talent level in IndyCar is easier to be compared within the series than this is the case in Formula 1. The clear structures of how well a team performs vanished with the late drastic rule changes. Now you have different drivers in different teams on essencially the same speeds.