PC. i think anyone who doesnt play a game like oblivion- that is, a readily moddable game- on a PC is a idiot. at least half of the fun of these types of games are the many patches and mods avalable- reballances, fan items and models, quests, areas, and even up to entire campaigns. I mean, look at the oblivon mod http://www.nehrim.de/indexEV.html
Its not a mod. its a entire coversion built on the oblivion engine. Thats -AT LEAST- two games in one, not considering all the other mods and quests out there.
There is no doubt in my mind that Skyrim will have such support when it comes out. And those poor fools on PS3 and XBOX360 are going to miss out.