If price cut and more launches Xbox One should get 15m easy. Current situation with only 13 countries, then the Xbox One will be fighting the Wii U for the 10m mark. PS4 has most countries launched and Japan next year should get 5m quickly, but idk about 20m before either get 10m...
I think Sony will react quickly to the Xbox one price cut.
PS4 - over 100 millions let's say 120m
Xbox One - 70m
Wii U - 25m
Vita - 15m if it will not get Final Fantasy Kingdoms Heart and Monster Hunter 20m otherwise
3DS - 80m