Revolution. First of all, SSB3 is going to be a release game, and it's also going to be out-of-the-box online, which is more than awesome. And from reading faq's and interviews it looks like the Revolution is going to be the cheapest. Not to mention the big mystery which is supposed to revolutionize the way games are played.
Xbox games are all the same, and they don't appeal to me so bleh (believe me, i've seen all the announced games). Except maybe Gears of War, but theres rumors that might be released on PC.
PS3 looks cool but it also looks very expensive.