I know this prediction might make me look stupid in a couple years but whatever. Here're my reasons:
I feel that the 3DS has reached its sales peak already last year/this year, it's downwards from here on, I see the 3DS selling more in the PSP range than the DSs.
I predict that the Wii U will not suddenly take off and post huge sales, I see it selling in the GC range.
Initial demand for the PS4 seems huge, so I see higher sales than the PS3 thanks to better pricing, smarter release and a way better image from the start (the PS4 is not "a bluray player with no games, that is way to expensive"), so I can see the PS4 selling over 100m units maybe even closing on the PS2.
Of course there are still a ton of variables here, for example the development of the mobile/tablet market might eat into the PS4s sales, maybe the Steam Box and of course the X1, maybe Amazon or Apple decide to enter in the race, maybe the Ouya (ok, maybe not :D). So that prediction is very dangerous, but anyway what's your opinion?
Prediction: The PS4 will outsell the Wii U and 3DS combined
Apparently I won my bet with prinz_valium