go for the 1600, reading that review which manigma posted, they are using the same silicon as the R7, so the 1600 R5 will be like the 1700 R7, it'll overclock easily to the 1600X speeds.
As for cores, make yourself future proof, games and apps arnt going to hang about on quad cores for much longer when theres so many multicore CPU's about now at reasonable prices, I recon by the end of the year we will see games taking advantage of more cores.