Both of these games are sequels, so I think it should come down to which did more for their respective franchise. Dead Space 2, which is by no means a bad game, still follows the formula that the first game did. While that could be said about ACII, the difference is that one of Assassin's main selling points isn't the environment and psychology. What made the first Dead Space stand out so much was that it was something that we don't see often see in North American markets (as tdigle alluded to earlier) and it provided such an immersive experience. Some of that magic was lost though on the second go around. It's similar to what most people complained about with the second Bioshock: when you play the first everything is so unique and mesmerizing but it just doesn't have the same effect when re-created in a sequel.
Assassin's Creed II managed to take a popular game franchise and turn it into one of the biggest names in gaming right now and became a house hold name so to speak thanks to ACII.
The first Dead Space was better than the first Assassin's Creed, but ACII did more for its franchise than Dead Space 2 did for its own.
Vote for Assassin's Creed II.