Multiple different and conflicting arguments taking place here...
In raw numbers of units sold, the smart bet is on the X360 winning the race. There are simply far more X360 consoles out there, so the initial potential consumer base starts out being much larger. And for the people who own both consoles, only being uninformed or having a strong PS3 bias could lead one possibly choose the PS3 version: between achievements and exclusive downloadable content, the PS3 version is inferior out of the gate.
(Some have expressed that they don't care about downloadable content unless it's free; well, you're in the minority. Companies are making major coin through downloadables these days, so clearly a lot of people have no issue with it. Even people who don't actually buy the add-on are better off if they at least have the option to do so should they change their mind. And I would further ask whether you have ever purchased an expansion pack for a PC game...exact same thing, except one is in a box and one is a download.)
The second question is which console will move more units from people buying a console just to play GTA. I think that that is likely to be the PS3. Because of it's "first release" on the PS2 for so long, GTA remains well established as a Playstation brand more in most people's minds. If these next-gen console holdouts haven't been swayed to buy an X360 by now, what with the decent price point and the frickin awesome game lineup from 2007, I would consider it unlikely that they would gravitate towards Xbox as their console of choice for GTA4. They probably have been waiting for GTA as the excuse to buy their PS3, and that's what they'll buy.
My predictions anyway, for what they're worth.