It's a good question, 6 physical cores vs 4 physical cores + 4 logical (Hyperthreading)
Physical cores do beat logical but the extra 2 threads will come in handy in some games. Games that don't fair well with Hyperthreading the 6 core 8600k will beat. Games that do support Hyperthreading the 6700k will have an edge.
Why not go for the 8700k? Will certainly last longer than either, a lot longer.