Assuming the same clockspeed there is not much difference in performance between 4th & 5th gen Intel CPUs. 5th gen "u" model CPUs consumes less power especially compared to the "H" and "HQ" model CPU used in the Y50 series. Especially since those CPU have higher clockspeed and "HQ" model CPUs are quad core CPUs.
CS:GO is a pretty easy game to run so if you like to have a 17" laptop with a 840m then the Z70-80 should be fine.