Are these used machines? If so, find out the GHz of the Y50.
Even then, while I'd normally lean towards the Y50 for the better graphics card, a quick Google search tells me that apparently there aren't any mobile i5's with quad-cores. So, the i7 is a better choice not only from more cores that video rendering/exporting can use, but also the i7's can hyperthread, more or less acting as 8-cores (quad x2), thus increasing your rendering capabilities.
General note, make sure the laptop you do get CAN be upgraded to 16GB RAM.
If I'm posting a question, it means I've Googled it for far too long and couldn't find a solution/answer.