slamgamer8 :
I am leaning towards the GTX 1060 since its $60 cheaper and has a significantly higher clock speed...
You can't directly compare clock speed as a measure of performance between different models of graphics cards, especially when it comes to GPUs built on entirely different architectures. That only can be considered relevant when comparing two cards of the same model (for example two GTX 1060s). The architecture of a Radeon GPU is considerably different, and these cards have different quantities of differently-designed shader cores, among other things. An RX 580 might be clocked lower than a GTX 1060 6GB, but in general the RX 580 should perform very similar on average. Each card will be a bit faster in some games and a bit slower in others, depending on which architecture a particular game prefers. The 1060 should be a bit more energy-efficient, but in terms of performance the two cards should be very much competitive with one another.
Of course, $60 is a pretty big price difference just to get Freesync. If you hadn't noticed, the prices of these cards have been very messed up recently due to shortages caused by cryptocurrency mining. Normally, the RX 580 8GB and GTX 1060 6GB would be more similar in price, and normally both of their prices would be much lower. When each of these cards launched, they were generally priced in the $250 to $300 range. Now, the least you can find them for tends to be well over $400. It's very possible that prices will work their way back down over the coming months, and it's even more likely that AMD and Nvidia will each launch a new generation of cards in this performance range in a matter of months, so it might be worth waiting if you don't have a pressing need to get a new graphics card right away.
It's also worth pointing out that the 1060 3GB has 10% of its cores disabled compared to the 6GB version, so performance will generally be around 10% lower on that version of the card, even when the lower VRAM limit isn't a factor.