User blog comment:IamJakuhoRaikoben/Sorcerer Magazine, Issue 45: September 2015/@comment-25111584-20151003185436/@comment-25324252-20151004060117

Erza is more powerful lore wise. The story and everything in it says Erza is stronger than Natsu. However Natsu is more powerful when the plot demands him to be. Natsu has plot force, Erza has plot armor. There is a reason Erza is an s class mage though and Natsu is not