Jenny Simpson: World Champion, 2011. Second best in the world, 2013.
Shannon Rowbury: Third best in the world, 2009.
The results speak for themselves. Jenny is a much better runner.
"Much better"? The only much better runner right now is Dibaba. At the world class level there's very little difference between competitors. Nobody is "much better" or "dominates" other world class runners consistently except for all-time greats. Even at the US level there's constant change at the top. '08' - 10: Rowbury, 11-12: Uceny, 13-14: Simpson, 15?