Performance are similar between the 3 and 6 gig memory, but 6 gig one has a better resale future vulue and won't be affected into next Ethereum dag stuff that will block every gpu under 3, or 4 go of memory.
There is as well the memory manufacturer that you may find while downloading GPU-Z. You want samsung ( Best hashrate and best overclocking abilities )
You don't want the single fan versions of the 1060s, you would lost more than 10% hashrate.
If the 1060 has a 8 pin connectors instead of a 6 pins, it's a must.
Always compare overall Mhz in specs, take the one that has more of those.
The single fans can pull the same hashrates, and are often cheaper too
I don't know about resale - will we really be able to get much for our 6Gb cards after we've had them running 24/7 for a year or two? We won't want to sell until they aren't profitable anymore so I guess that timeline depends on Eth prices, but many miners are holding Eth anyway
As for the 3GB vs 6GB, here's some info on DAG file and sizes - you should be good to mine Ethereum on a 3GB for another year or so.
https://www.pyramidreviews.com/cryptocorner/mining-gpu-speed-and-price-comparison/ethereum-mining-speed-gpu-comparison-for-gtx1060-3gb/Lucky I guess, probably because of your Samsung memory.
On my 13 graphics cards, I got 2 single fan with 6 pins electricity sockets, and they give me the worst hashrate. I didn't checked their memory brand though, could be a good thing I check that eventually . I get around 5 mh/s less while mining myriad groestl ( stock, I usually get around 56-57 mh for my dual fans, and around 50-52 mh with my single cards)