Your argument is good when you make the assumption that everyone cares about PC gaming, but unfortunatly this is not the case.
Either way, I would argue that having both a PS3 and a 360 are redundant, since they have a very similar library. If you don't own a DS or PSP or Wii, I suppose it would make sense. However, if a gamer owned a DS and a Wii, it is likely that they would buy a PS3 or a 360, as they would probably not have time to play all the games available for 4 different platforms.
Playstation is not automatically superior. This is only the case for hardcore PC gamers and Sony fans. For the rest of us, game selection and other features, such as storage capactiy, Blu-Ray playback, online services ect must be considered. In my case I found the $300 20GB to be the best value as I want to play console games and a few arcade games.

For PC a gamer just a wii and 360 is cost effective and gives one of the best gaming experience. Add a DS and PSP there is almost too many games to play. Having both a PS3 and a 360 is redundant since their libary overlaps too much save for some exclusives like MGS4, FF13vs and GOW3.

ff7 was a game for adults? ....................