I'd probably recommend the Z170-A because it has almost all the features of the Pro Gaming model. Although, if you're using many peripherals, the extra USB 3.0 will be helpful to you. But it all depends. If you're using it for gaming, go for the Pro Gaming model (as it's enhanced for gaming). If not, there's no real point.
TL;DR
If you're using it for gaming: Asus Z170 Pro Gaming
If not: Asus Z170-A
I'd recommend the Asus Z170 Pro Gaming.