Guess work at the moment isn't it. Popular characters, like Scorp become good, because so many people play with them, so they get better at using them.
In the demo JC is best for my style of play. Fast with some good combo's and he has the Sonic Boom type kick to beat people jumping in.
But many player who can do 50%+ combos will probably like Sub Zero and Scorp better as they get to unleash them easily when opponent is frozen/speared.
I personally could not get on with Mileena, but others love her.
I hope the game has decent balance, but I also kind of like games to develop where certain characters are stronger and weaker against others. They have a Nemisis. This adds to it all IMHO - and to try and remove that means you get a load of characters with a move each that does the same thing....that is dull.