The digital upgrade version
Ultra Street Fighter IV is out this week, and given that there are multiple iterations of the game at this point, you should watch the trailer to see what you're getting. Namely, the new characters, new stages, rebalancing, and new battle mechanics.
The video also shows some crowd shots from various matches within the fighting game community, which is a nice touch. You can upgrade today with an existing copy of Super Street Fighter IV or Super Street Fighter IV: Arcade Edition.