regarding the looks, the Z400 will be thinner compared to Y400. for the keyboard, both have backlit keyboards but Z400 is white coloured while Y400 is red coloured. screen quality is the same as both are 1366x768 HD resolution. for heat, the heat produced on Z400 will be greater than Y400 due to its thin size. adding on batteries, the Z400 battery is non-removable but Y400 do. Y400 also supports a swappable bay for dvd drive, hard disk bay, extra graphics card for better gaming performance or cooling fan.