There isnt much price difference in both. And the performance gains are significantly good. Z170 motherboards have better Power delivery to the CPU than a Z97 , so overclocking on Z170 will be awesome. DDR4 RAM has 10-15% performance gains over DDR3 RAM which means around 6-8 more FPS. Also they are newer generation with latest features. Thus DDR4 build is better for sure. No doubts regarding it.
Also if you are planning to buy a GPU then i would recommend you to wait till AMD's Polaris and NVIDIA's pascal cards are available in market.
Good Luck & Happy Building