From a gaming perspective the Y50-70 is much better than the Z51-70.
It has a quad core CPU vs. only a dual core CPU which can come in handy for games capable of using more than 2 cores. But the nVidia GTX 860m is also a lot more powerful than the Radeon R9 M375. That accounts for the $210 difference based on Lenovo's website. The Radeon is a little less powerful than the Geforce 840m which is considered a mainstream performance mobile graphics chip.