Well above all, ask yourself which you want to play more. If you're okay with a so so shooter, then Dead Space 3 is your choice, but if you rather test yourself with a fighter... then Injustice.
With that being said, Dead Space 3 is pretty terrible. They kept a lot of the rigid gameplay from the earlier more survival horror versions and tried to make it slightly more of a tactical shooter. Despite what people said before and what you might think, you won't miss out on a lot by not playing 1 and 2, though don't expect the story to make complete sense to you. However, I felt that 3 had a terrible story and the gameplay was only so so. Now a days I don't suspect many people are playing online and the ones who are, probably have the devil horns and will over power every situation. Also there isn't a lot of replayablity, since the side modes suck.
Injustice isn't the best fighter, but it's certainly solid. The addition of SF controls was a welcomed addition and the future DLC doesn't look completely awful. With all the sales and strong lean towards fandom, it is without a doubt better than Dead Space 3.