As was said, 6GB is plenty for gaming, however, I wouldn't take either of the sets you listed, 12GB is overkill, Corsair Dominator is very over priced, all you get is a fancy heatsink and "higher quality" chips than standard XMS3, the quality is so much better that stock performance is identical and overclocking is just as good on XMS 3as on the dominators...
But I would take neither the XMS3 nor the Dominators, I'd take G.Skill Ripjaws:
Newegg.com - G.SKILL Ripjaws Series 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL8T-6GBRM
Much cheaper, same frequency, same timings and the quality is still excellent. So far as performance and overclockability, the same, often better than the Corsair