In advance: Sorry for the upcoming wall of text. I just summed up ALL of my thoughts on the topic.
1) Tournament-wise UMVC3 stands above MK9 in terms of everything: hype, money, participants, competition, fan-base.
There is no doubt that UMVC3 is superior to MK9 and that the Injustice hype finalls starts attracting more and more people from other communities, which MK9 did not do. And what was neccessary for that? Vast changes to the mechanics in the game.
2) The Capcom pro players already have their focus on certain games, very often on at least 2 or 3, because there is the most money in these games.
I want to see MK9 pro players playing multiple fighting games at the same time and making a name for themselves over and over again. No, they stick to MK9, a game that vastly differs from many other standard fighting games on the market. It's easy for you to talk trash and call other people bad for not being able to own in MK9, while you wouldn't do any better in their games.
3) Capcom games have much more talented players in the community... Let's face it :-7 One does not simply compare himself with Daigo, Infiltration, Justin Wong, Chris G, Combofiend, MarlinPie and so on.
Out of all the Capcom players FilipinoChamp is the one who considers himself as a not very talented person. He said that he has to practice on a daily basis for hours to be able to compete with the others. He sacrifices a lot for UMVC3 and that brought him a win in EVO 2012.
If he picks up Injustice and takes it seriously (which might not happen, because chances are high that he will continue to make more money with UMVC3), then expect him to probably put more effort into the game than any other guy in this community.
4) Ultimately ... It's kinda a shame to see the reactions from the people here. This is now way past saltyness and stuff like that. The stuff that goes on in this thread is sheer stupidity and that's indeed a pity. It's actually something you would expect from SRK's front page, yet we have it now here as well.
The biggest wish from NRS with Injustice is probably to make the game more accessable to other communities. That's also the reason why they try to cover up a lot of options to make it more interesting for other players, so that they would have a game that feels more familiar to the stuff they play (Capcom fighters obviously)
Yet you people question and trash-talk their skills. And it's not a simple calling out right now or something like that, you're calling them bad and insulting them, because they were not able to pick up MK9, a game that is vastly different than everything they played before and that offers less money and less sponsors than the games they usually play.
You all remind me of Nick Diaz. He is a good mixed martial artist. In the ring he can outbox a lot of people, but as soon as they take him down, he starts bitching about MMA being shifted towards wrestling.
It's the same story now. The Capcom players would punch the living crap out of you, but as soon as the fights goes to the ground (I hope you realize how I used that example to also showcase where MK9 stands compared with Capcom fighters), they don't know what to do and you start talking trash, because they don't know how to fight at the ground, yet at the same time, your stand-up is pure garbage.
Injustice will combine the two worlds. The only similarity I personally see in Injustice and MK9 is the combo system, which is not purely based on hit-stuns like basically most of the Capcom fighters.
If Capcom players manage to get past that and pick the game up well, then damn ... expect their talented people to beat your pro players.
Also as a note: I'm not biased towards any community. I'm just hinting out ignorance. No offense.