Skylake is much faster than Sandybridge in gaming guys.
Most who say Sandy is still fine are just fooling themselves.
Skylake is a good 10% faster in games than Haswell, especially in games that aren't well threaded.
Take a look at this, for example:
Who cares?
The CPU won't be your bottleneck for a long time. The extra cores in the 5820k plus the lower price make it a better buy for that reason alone. If you don't need that then you stick to 4 core haswell.