Well, in general, to figure out 'faster' a quick and dirty way is to divide the mhz by the cas speed. This gives you a rough number to compare different speed/cas setups.
For example, you have 1866 c10 ram which comes out (conveniently) to 186 'score' for $154.00
Gskill also makes another set of DDR3 (Ripjaws X 16gb kit) which is a 2133 cas 9 which gets a rough score of 237.
That puts it about 25% faster for 6% more money. Here's the link for it at newegg
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231571
Of course, you have to make sure your motherboard will support 2133 mhz memory speeds, but if it does I'd go with the Ripjaws set I linked you to and it (IMHO) would be no-brainer (again assuming mobo support). One other quick note, because the 2133 c9 is the max speed it can run at, it can certainly run at a -slower- speed, and might have a corresponding lower cas timing than 9. If that's the case, if it runs at a (just a wild guess) 1866 cas 8, you'd still have a higher score and would still be signficantly quicker than the 1866 cas 10 memory you're looking at.