I've already said that I'm getting the PC version since I"m not a big GTA fan anyway. If there is no PC version, then I guess the DLC might be a small factor in which version I get.
Most likely, the DLC won't matter to me as much as better graphics & loading.
I just wish that the PS3 could upscale to 1080p as well as the 360. The upscaling differences between the 2 systems are pretty noticeable on my 24" monitor.
Recently, I've been console gaming alot more on my monitor than on my TV so the 360's versatile scaling has been quite useful.
Also, your comparison of PSP ports to the 360 DLC isn't very relevant because those ports were going backwards from San Andreas while the DLC is using the same tech as the core GTA4 game.