What is the better opportunity? Super Bowl ratings are huge and performers always see a nice increase in sales. However compared to the World Cup I think the Super Bowl is just too local.
Super Bowl. A World Cup performance doesn't garner the same attention or isn't considered a big deal like SB.
Let's put it this way, it's not a SB without a halftime show, people actually anticipate the performance, whereas you don't really anticipate a WC performance.
World Cup performance is less memorable eventhough it's watched by more. But not by that much more... the Super Bowl performance gets about the same amount of viewers as the sport. Can't say the same for the WC pre-show.