Because each may have a strength in some area such as the quad core may excel at video editing and the 4GB graphics may have the edge at games, there is no clear winner.
It could be strictly a budget issue. So the lowest price wins.
I am in a confusion whether to buy lenovo z510 or z50. Both are just having 2000 rupees difference. First one is coming with i7 quad core , 2 gb graphics and second one with i7 dual core, 4gb graphics. Please suggest me the best one.