Impossible to say. I believe Kubo stated in an interview (or maybe it was in the databook MASKED) that Espada ranking was based on pure reiatsu - not special ability or anything like that. That would mean that Yammy had higher reiatsu then Starrk. But we all know that unless the difference in reiatsu is astronomical, i.e. an unseated shinigami against a captain class shinigami, reiatsu difference really doesn't matter. So if they were to fight, it would come down to who had more skill in fight.