Mira and Gray heads over to the Erza and Jellal thread to stomp them then make out to celebrate their epic victory
I think both Gray and Mira have been two of the most likeable and unfortunately, neglected characters. In Mira's case especially, she hasn't had much on-screen feats. If it's up to me, I would hope they both advance. But since it's come down to this, if I have to pick one to advance then my votes goes to Gray.
My decision doesn't involve guesswork around questions like Gray should win because Mira is vulnerable to demon slaying magic, or Gray has already beaten Mira along with Elfman. Or Mira should win because she and Elfman could stalemate Skullion and Madmole. If we remove the hype and portrayal and assess only their feats, Gray comes out top as the more impressive fighter if I'm being honest.
I have seen a few posts that Mira has the speed advantage because she can fly. But I think the flight advantage is overstated. There are even a few posts that claimed Gray is slow. That couldn't be any further from the truth. Gray is capable of very swift movement that allows him to travel faster than the eye can follow. He vanished without a trace then reappeared behind Bixlow and Evergreen who had their eyes locked on Gray and were several meters away, before either of them could even perceive wtf happen. The visual imagery is reminiscent to Mest's and Minerva's teleportation magic, except Gray could achieve it with his raw speed.
It happens that Ever also has the ability to fly too.
Gray has the reflexes to evade fast-moving attacks such as a laser from one of the god seeds who powered up after absorbing Juvia.
He could blitz Invel and Skullion with zeroth longsword before either of them could react despite knowing the attack is coming.
Skullion himself is fast enough to evade multiple attacks from an enraged Mira.
And eventually moved behind her to land an attack without her noticing in one exchange.
Yes, I know that is Mira in her regular Satan soul takeover. She has other more powerful takeovers such as Sitri and Alegria. Unfortunately, Mashima did not bother to give her any on-screen feats with those takeovers since the days it was used on the likes of Jenny, Juliet and Heine. And those characters are feat-less. For all we know, Mira could and likely perform better with her more powerful takeovers. What we do know, because we have seen it, is Gray has better displays of speed. It's shrouded in less ambiguity. Flight is a useful ability to have. However it isn't the be-all, end-all. It is a means to an end. There's a good reason why birds, despite their ability of flight, remains a diet for big, hunter cats like leopards, cheetahs, jaguars and etc. If you are equip with the necessary speed, skill and weapons, you can still take down targets that can fly. And someone of Gray's calibre possess those traits in abundance.
He is an expert marksman who is very dangerous from range. He perfectly pin point and snipe foes who were at a great distance from himself.
Yes, I know Natsu and Lyon were there at that time. However, if you read, both of them were clearly no longer able to fight at that point of time. They were sitting ducks who did not put a resistance. Both Racer and Mard had the time to notice the attack was coming, but still failed to evade them in due course. Again, Mard has the ability of flight too.
If we factor in PoF then Gray becomes considerably more powerful. Gray lost to Skullion because there was no PoF involved. At the height of his powers, Gray can ragdoll a Spriggan like how deliver a brutal pummeling to Invel. He could then go on to stalemate an enraged END Natsu. I don't think we have seen Mira attain such heights in the story yet.
Mira, in her own words, only managed to pull off a victory over a Spriggan like Jacob due to his self-sabotage for fighting with his eyes closed, hence he could not see anything.
END Natsu was the strongest demon in existence. Stronger than Larcade, who is hyped and portrayed as the third strongest Spriggan. Larcade could block a barrage of sword strikes from Kagura. And Kagura isn't as outdated as you'd think because she could cut down down Wall's attacks and react to Dimaria's attack that drew a compliment from her.
If Larcade is a lot faster than Dimaria and Kagura, it made complete sense that END Natsu blitzed and massacred Dimaria. Gray could clash on equal footing with END Natsu. And this is a level of strength that could easily wreck and decimate any Spriggan not named August or Irene.
Gray has incredible AoE with Ice-Make Silver. The area that he froze are as large as Mira's blasts and he should have an excellent chance to do so.
Gray also has zeroth destruction fists and Heavy Knights that are very hard-hitting and looks a class above what Mira had achieve.
He has moves in his arsenal that could overwhelm her, but I don't feel the same could be said the other way round.
Mira's Macro revolves around mental subjugation so her targets have to listen to her command. However, Gray has shown remarkable mental resistance when he broke the Genjutsu of his perfect world that Hakune placed him under.
While the Alegria takeover that Mira used to beat Juliet and Heine heavily exhausted her after a single attack in that form.
Mira has to defeat her foes with one attack in that form. Against somebody like Gray, who along with Natsu, has the most endurance in FT after Laxus and Erza, spells doom.
Gray could effortlessly freeze multiple pillars of END Natsu's flames that erupted from below the ground around him.
So the flames from Sitri soul are likely not going be a huge threat to Gray despite his elemental weakness to them.
The more I ponder about it, the more it seems this fight is a bad match-up to Mira because Gray's skillset is almost tailor-made to counter everything she can offer. It dawn on me while typing this post that the gap between them aren't as narrow as I initially thought
. Jacob fought Mira the same way he fought Natsu so him not using his hax is a moot point. Also literally none of the S12 use their hax to the fullest, this was shown countless times. If you want discount Mira's feats then we could do the same for Gajeel, Gray, and everyone else in Alvarez. Jacob wasn't nerfed in any capacity. He also literally stated he can see presences which is how he was landing shots on Mira. Closing your eyes also doesn't decrease stats in the slightest. Not being able to use your full MP = a nerf, not using using your eyes or hax =/= a nerf. Only person who was nerfed was Mira. 
go go Mira 
