The X360 is facing market saturation which is causing its sales to decrease. The PS3, however, doesn't have market saturation yet and is still experiencing good sales.
My prediction threads:
Wii U will sell under 40m units (made on 14th September 2012)
PS Vita will sell under 20m units (made on 30th September 2012)
Wii U will sell under 7m in 2013 - I was right