2500K for sure. Most games can't even use 4 processors, let alone 6, and the 2500K outperforms the AMD in just about every way, but especially in gaming.
I also need help in deciding RAM. i5 2500k supports maximum of 1333Mhz, is there any advantage of using 1600MHz RAM. Is there any quality difference in performance difference of different RAM manufacturers. Can you recommend me a 2x4GB RAM of CL9 type(CL7 is not available)