Obviously faster = better... BUT, I'm assuming the faster RAM is going to cost you more money? If not, get the fastest of course.
The real question is whether it's worth paying more for the faster memory. For most use-cases memory is very rarely a bottleneck, particularly with Intel builds where the Intel memory controllers are mighty-quick. Once you get to 1866mhz, you're usually better investing the money you could have spent on faster memory into other components where it will make more of a difference.
Do you have any abnormal uses planned for the computer? Are you just gaming, watching movies, browsing, etc? If you're into content-creation you could make a case for spending more on RAM, but otherwise, put your budget elsewhere would be my advice.