User blog comment:BlackWizardZeref/Fairy Tail Chapter 399: Wings of Despair Review/@comment-203.111.234.105-20140909162133/@comment-108.28.233.191-20140910063809

You know, I'm fairly sure that Mashima's never really said that Erza and Mirajane were evenly matched. Especially in present times. Erza was introduced as the strongest woman in Fairy Tail, and nothing since has really been done to challenge that.

Yes Mirajane and Erza were "rivals" when they were kids. Said rivalry seems to have consisted primarily of Mirajane picking fights with Erza. Erza however explicitly made S-Class a year before Mirajane did, and Mirajane was only active as one for around a year before the incident with Lisanna happened and Mira retired from taking jobs and basically stopped working as a wizard, effectively stagnating herself at the level she was at when she was 17. She regained the ability to fight, but doesn't seem to have done much training despite coming out of the fight with Fried looking like a mess. Her stagnation was amply demonstrated on Tenrou Island where after the fight with Elfman, she lacked sufficient magic to properly fight Azuma and lost. Whereas despite fighting Juvia, Erza had ample magic to fight Azuma, and still go on to fight with Hades. Mirajane partially mitigated her problems with the training before the GMG, but Erza improved at the same time. We see those results in that Mirajane's out after one fight, while Erza's still going strong.

The bottom line is that throughout the story, Erza's been unquestionably portrayed as the strongest woman in Fairy Tail, and even if they were equal as kids, Mirajane will have logically fallen behind years ago, and still be playing catch up; while doing little to actually catch up.

They pretty much get equal respect from the guild, but that's about it. Erza's #1, and Mirajane's #2.