Why can't it be somewhere in the middle?
The Wii competes with the PS360 for the core gamer market. The competition in that market is more even between the 3 consoles.
The Wii also has a monopoly in the casual gamer market. This market consists of consumers that would NEVER consider a PS360 and this market is much much larger than the core gamer market... (and it will piss me off if someone says that all people who buy the Wii would consider the PS360)
PSN ID: T_Gears
End of 2009 ltd sales:
Wii = 67-68m
X360 = 38-39m
PS3 = 34-35m
Prediction: The PS3 will surpass the 360 on weekly sales after it drops to $299 on all regular weeks (no big releases).