Seriously, how would SF characters work with tekken gameplay? Tekken x SF seems to be the one that's not a straight up cash in here.
They would likely remove the projectiles and give them normals/specials that are similar to the characters' supers.
Look at Kazuya. He has a "shoryuken" and "hurricane kick" in the game. Look at Paul. His smashing fist could easily be used in place of a fireball.
Without knowing anything, it's a good assumption to make that if you like SF-style games, you'll want to give SF X Tekken a chance, and if you like 3D fighters, Tekken X SF will be your thing.
The two games are going to be completely different from each other. They even said that the two teams are not sharing information with each other and are free to change up the character designs as they see fit to. I wouldn't even take the demo shown as any kind of indication that's what the final game will look like, graphically. It's not supposed to be out for like two years (damn!). Ono said that they are thinking of going with a more classic character roster (read: SSF2T Vs. Tekken 2) but the Namco crew could go a different route entirely. Since (S)SF4 already brought back all the SF2 people (again), that's already a little disappointing, but I guess this game could end up simply being "SSF4 Turbo" anyway.