The performance difference between the Y40 and the Y50 is enormous. The Y40's i5-4210U Processor is similiar to an i3-3000M series Processor. The "U" designation trades performance for battery life effiency. So the Y40 should have more stable battery life at 4-5 hours compared to the Y50.
However, the Y50's i7-4710HQ and 860M is miles above in performance compared to the Y40. The Y50 isn't much heavier with the weight of 5.3lbs and the Y40's 4.9lbs.
The screen size is the only difference in physical attributes. So if you have the extra $200, the Y50 is the obvious choice and the difference will be night and day.
Let me know if you have any questions!
