MK9:
-Dash cancelling
-Block button
-Overall brighter, and aesthetically pleasing graphics
-No trades
-Tag team
-Unlockable characters
-Contains more uniqueness in gameplay, where as Injustice is more of a combo of several games.
Injustice:
-Better details on certain areas, such as the pupil's on the characters.
-Better hair physics(still not great, but definitely better than MK9).
-Character trait button
-More interesting Story Mode
-GREAT amount of DLC
-Cycles through alternate costumes in Arcade Mode(s)
-Several alterations of Arcade mode, giving good Replay Value
-Slightly better Online(still rather mediocre though)
-Cool UI on the character Select Screen
-WBID, and the unlockable content connected with the iOS APP.
Overall, I like MK9 better but Injustice is absolutely an AWESOME fighting game, and I hope there is a sequel