At the end of the 2015 season, LRC ranked Rowbury has #1. They were both pretty much even at the end of 2015 but Rowbury was given the upper hand based on the fact that she broke the American Record. Now in 2016 who would you say is the best woman. Not just this year as a whole but in all their years included.
Personally, hands down that goes to Jenny Simpson. She overall just has more medals than Rowbury, is more consistent, and besides 2012 has done something exception every season.
Although a number of you would pick Rowbury based on the fact she holds both 1500 and 5k American records, however IMO records are nice but they can always be rewritten. Rowbury just has the records until the next lady comes along and snatches it. However, Simpson's medals can't be taken away. Her name is attached to each of those medals for the rest of eternity. Rowbury's name to those records is not indefinite and can be easily removed within the next coming years.
Medal/major championship comparisons between the two
Jenny Simpson: 2011 World GOLD, 2013 World SILVER, 2016 Olympic BRONZE, 2014 Diamond League Title
Shannon Rowbury, 2009 World BRONZE, (Not counting her 2016 World Indoor bronze, that was at a different distance (3k) and a very less competitive field)
Simpson has more national titles as well
2014, 2015, and 2016 1500m wins at USA in addition to 2013 win at 5k
Rowbury only has a 2009 1500m win to her credit. Even if Rowbury wins any medal of any color at 2017 Worlds she still would not compare to Jenny Simpson and her accomplishments.