The release date for the game will be the same obviouslly... but don't be so sure aobut the DLC. It wasn't just Oblivion that had exlusive DLC as both Fallout 3, and Fallout New Vegas had 360 exclusive DLC at first. We now know the NV DLC will come out for everyone at a later date, but its a pretty solid track record by Bethesda.
Well truth of the matter is the Xbox 360 is just easier to code games for. On top of that, Morrowind was on the original Xbox, so there is some history between Microsoft and Bethesda. On top of that, Microsoft just knows how to run a business better than Sony. Aside from the whole Red Ring of Death incident, Microsoft has definitely been beating Sony every step of the way. The only thing that has kept Sony alive is their few exclusives, and their complete turn around of their horrible business tactics at the launch of the PS3.
A quick break down of the pros and cons of the Xbox 360 and PS3:
Xbox 360:
Pro - Better exclusives and overall better games. Better online service (have to pay though).
Con - Horrible hardware (RRoD and no bluray)
PS3:
Pro - Better hardware (still pales in comparison to PC, but has bluray). Free online service (PSN still isn't as good as Xbox Live though).
Con - Less exclusives and overall less games (although Sony has been working hard to turn this trend around).
Overall, Xbox 360 usually has the better games and a better online system. But PS3 is generally a better console and has a free online system.