Well really it depends on what you are going to be doing with the ram. Gaming? Photoshop? Just using the internet? all of those things contribute but in the end for like 2 bucks difference I would go with the fast 1600 ram and you can always tighten the timing.
I will mainly be using it just for gaming, but I will be using basic programs such as Internet, I tunes etc.. No photo shop or anything along those lines.