BIOS update is not hard, but it requires 6th gen cpu to do it. If you don't have 6th gen cpu, you can't update.
i7-6700 is 6th gen, so no update necessary.
But if you have to choose between i7-7700k and i7-7700, then price difference is important. If 7700k is significantly more expensive, then choose 7700. They are basically the same, main difference is - 7700k can be overclocked on appropriate motherboard (not on AsRock H110M-HDS).