I've used both these boards in the past so I can try to give some insight.
First off the ASUS Z170. I preferred the BIOS UI on the ASUS motherboards over any other motherboard manufacture. It's just a lot sleeker to me and everything is where you need without looking clunky. The motherboard ran a little cooler than the GA-Z170X, probably about 5c. I used the exact same case and fans, in the exact same configuration and noticed the difference in temperature. Every ASUS motherboard I have owned comes with a Q connector for the front panel connectors, which make it a little nicer looking down there. MSI also includes their M connector. I haven't seen any sort of connector from Gigabyte. The ASUS mobo also has an onboard reset switch, in the instant that something happened, your case button isn't powering it off, you can simply open the case, press the button and it resets. It also has a clear CMOS switch, in case you mess something up in BIOS; instead of removing the CMOS battery for a few minutes, you press the switch, and wait like 30 seconds, power it back on and your BIOS should be cleared completely. The M.2 port on the ASUS is in a better position than the Gigabyte in my opinion as I believe the Gigabyte M.2 port is very close to a case standoff hole.
Onto the Gigabyte Z170x. If you care about aesthetic looks, such as colour, the colour looks a little better on this one. Red, black and white looks a little better than blue, black and grey in my opinion. The USB 3.0 connectors are in a better position and right next to each other. On the ASUS the USB 3.0 connectors are not even close to each other, which I had to sacrifice using USB 3.0 for my front panel because I couldn't reach it. Your case is different from the one I used, so it may be different from you. The Gigabyte motherboard supports RAM overclocking up to 3733 MHz where as the ASUS motherboard only supports RAM overclocking up to 3400 MHz; not really an issue for most people at all. If you plan for more storage, the Gigabyte has 13 SATA ports where as the ASUS only has 8 or 9 SATA ports.
When it comes down to, I didn't notice anything different performance wise that really mattered. The ASUS booted a littler faster, but it had to barely be a second faster. Both ran my hardware no problem, I had no hiccups, nothing failed, nothing was being held back. I kept both those motherboards until I moved up to X99 motherboards about 8 months back.
There are a few things to consider that will make the choice for you. Do you need that many SATA ports for storage, do you care about how far you can overclock RAM? If so, I would get the Gigabyte.
Do you care about slightly cooler temps on the motherboard? Do you care about having more internal connectors and switches? If so, I would get the ASUS.
Other than that, if you care about colour or how the BIOS look, pick what you like.
I will say though, you may need to get some sort of fan hub. I think both motherboards have only 3 case headers, not including the CPU fan header; which is under how many case fans you wish to put into your case. Also ditch the sound card, both motherboards have their own built in sound card.