-What CPU's are you looking at?
-2 as mentioned above it depends on the game.
Chances are the Dual Core doesn't have hyperthreading (with exceptions in recent Architecture), which means if the game makes use of hyperthreading and multi-core processing which most modern AAA titles do, then the Quad Core will be superior. If the game is older or less optimised and runs on individual core speeds then the dual core would be better.
All in all i would always recommend choosing a quad core at minimum for gaming if you can afford them. The only major exception is the new Kaby Lake i3's as they are Dual Core hyperthreaded gaming machines.