Your question has very little merit because the conditions given discard most of the distinctions between the devices. If all games are avilable, all gimicks are available, all additional content is available and they are all free, then for the purposes of the question, there is little upon which to base a conclusion.
That said, my choice would be a PC. I have ALWAYS prefered playing games on PC and, on the occasion that I do not, it is because the softare is not available or my social circle has chosen to play on a console.
Of course, if we look at the consoles themselves, the reasons for purchasing one platform over another have essentially disappeared. All we have left is basic controller design (where I prefer the 360), network (the 360 is the most robust and the least free, PSN is free but less robust than XBL, nintendo's network can be said to exist and is also free but I have thus far seen little value), and value added features (PS3 wins because of blue ray drive, 360 is next and Wii comes in a distant last since there isn't much it can do besides play games).
RTS fails on console most of the time. And FPS are better on PC. Plus, it's much easier to mod games on PCs, since people just create the programs if they aren't already out, so there would be so much additioanl content. 
