which one is better?

Solution
The 6700K is cheaper than the 6700 right now, also note that the K series are clocked slightly higher at stock than the non-K series.

Supahos

Expert
Ambassador
6700k is the only one you can run without a bios update (most likely) if you have a skylake CPU around right now you can update the BIOS and the the 7700k of you choose.

The k in both of those CPUs means it is unlocked for overclocking and you're paying extra for that ability. However your motherboard will not allow overclocking. Those CPUs also do NOT come with a CPU cooler.

You can get them and buy a cooler, get 6700/7700 (without the k save a few bucks and get a stock cooler) or get either of the ones you linked.
 
The discounted price ($309) on the 6700K makes it the better choice, but the 7700K is slightly faster. I would get the 6700K at that price. Your board would need a BIOS update to support Kaby Lake.

I don't believe there is another CPU that comes as close to the 7700K in gaming performance as the 6700K.