The advertising you see depends on who is paying for it. If Rockstar/Take Two is paying for their own advertising, it will likely show you any systems it is available for. If one of the companies (Sony/MS in this case) wants to pay for it themselves and only show their system logo on it, and in this case tout exclusive features, then who is Rockstar to pass up free advertising? MS should be advertising extras if they've secured the rights to them. You know if Sony had DLC on the way, they'd be marketing the hell out of it too.
As to why I think the DLC marketing has been slow so far: It's still quite a ways off. Neither MS nor Sony need much to get people to buy the game right now. GTA sells itself at this point, particularly at launch. It will be as the holiday season approaches and the companies start looking to move more and more systems that MS will more than likely go full on with "We have exclusive content for GTA IV, buy the 360 for Christmas if you want to get GTA". My guess at least.
The dedication you show to any particular console or company is inversely proportional to the number of times you have gotten laid. If you get laid enough, even if you prefer a certain brand, you just don't give enough of a shit to argue about it on the internet.
