Simple answer: Higher speed will make more difference than lower timings.
The difference will be very small either way. It's really only worth debating if you are an obsessive benchmark person. In real game play, you probably wouldn't be able to tell the difference between DDR3-1333 9-9-9-24 RAM and DDR3-1600 7-7-7-18 RAM.
I suggest a 1.5v spec 8GB (2x4GB) DDR3-1600 CL9 kit, like the Corsair Vengeance or G.Skill Ripjaws / Ripjaws X. Spend your money on something that makes a noticeable difference, like an SSD or a more powerful graphics card.