Could go either way, I'd go the 16GB, more and more games are beginning to utilize DRAM more and prices aren't likely to drop soon, so get the 16 and then you already have it, will also be handy in multi-tasking, and other things you might do.
You star to get diminishing returns after 1600. Same as going over 8 Gb capacity for games. However, the 16 Gb will benefit more in the long run if you are doing multiple tasks at once such as having a couple of web browsers open watching youtube on one and gaming in another. If you run any virtual machines this can also be a great addition. There is not a super discernible difference between 1600 and 1866.
I would recommend G. Skills Ripjaws X memory. Looks good, functions good, runs at listed speeds, rock solid. I have had to deal with their customer service once because of a DOA stick and it was a great experience.