Is the fighting style of Street Fighter X Tekken be that of Street Fighter which is more fast paced or will it be more like Tekken which is more slower and more "realistic"?
The fighting style is similar in feel and speed to Street Fighter, but features character-swapping and character bouncing off the edges of the screen like Tekken Tag. Also like Tekken Tag, if one of your team members goes down, you lose the fight, even if your other teammate had full health. The fighting is all done on a 2D plane like Street Fighter, however.
Check out the video review at GameTrailers as they comment a lot about the engine itself.