I doubt Rockstar will regret taking $50mil off Microsoft, I dont think the DLC would have made anything like that even if released at the same time on all platforms.
TBH I still think this will seel quite well, its practically a totally new GTA game, albeit slightly smaller than IV itself.
For those who have sold GTA IV you dont have to buy the game back if you want to play the DLC, the DLC doesnt actually add anything extra to the original game with Niko as far as I know, it just adds two new single player campaigns - which can be bought on a seperate disc that does not require the original game to play. Looks like this will sell at around £20, so it will be interesting to see how much the downloadable versions cost.
The only thing thats the same as GTA IV is the city, the controls and the trophies that should appear as DLC for the game, rather than get they're own seperate list.
You can see the differences between the DLC and disc versions for the 360 here, but its basically that the DLC can use all the music from the original game plus some extra, whereas the disc cant so it has some exclusive stuff to compensate - and the online stuff should be compatiable between both versions as they will be realeased for PS3 at the same time. Obviousley the price will be a big influence on which version peeps get.