Most of this was over by fall of 2012
Wheras with Injustice, it was still hype to see Ducky play against Wound in the very last tournament. If Theo, Jupiter, or Sonic ever came back to face each other super late into the game, it was hype. Or if 16 Bit played King at any point in Injustice's life it'd probably carry a similar level of drama.
Mk9 deteriorated into Kabal/Cyraxfest (with the addition of Pocket Kenshi to combat Cage in 2013), which is something that despite the potency of Batgirl, MMH, and Zod, never happened in Injustice (You'd usually see max 1 Batgirl ever, 1 Zod making top 8, and 1 MMH usually).
Like, for yet another player top pick up Kabal, Cyrax (or later Kenshi/Sonya) in MK9 was no big deal because of the frequency -- whereas more of the top players remained character-loyal in Injustice and I think it made for better rivalries and matchups as the game's life progressed.
So I feel like MK9 had the strongest start, but one of the weakest ends. Injustice had a shaky start but quickly gained momentum and storylines that lasted all throughout.
It's an interesting comparison.