For one video card a 750watt would be enough but make sure you shop around for the best price. The price difference is only 25 dollars more or less anyway (between the two models you mention) so for futureproofing you could always go that route (larger PSU). 

RJ and Bob are definitely correct, the other thing to take into consideration is overclocking.  This can really increase power consumption.  In general, single card solutions and overclocking that is in the range of what is usually seen for 24/7 clocks a 750 watt is typically enough.  If you were going X99 and wanting to push the clocks hard on a CPU I would go 850, but Z170 is VERY power efficient in general.

Get one with Eco Mode switch - fans do not belong to category quiet. Other than that just think ahead and decide if you'll ever add more HDDs to the mix. Outside CPU/VGA combo HDDs are next in line for power. While nominal power for HDDs is fairly low you need to take into account that during power-up each HDD takes about 20W.
 
Between 750 and 850 I would pick the latter simply on the basis that you get fair safety margin if you ever decide to add another VGA or additional 3 HDDs. You never know. And on 100W difference PSU efficiency won't suffer - like buying 1500W brick to power CPU, VGA and SSD. LOL

Use this :) http://www.evga.com/power-meter/