In general, the 1080 isn't too popular. For the price and power use, there are better options. However, in some cases, the lower popularity does mean you'll be able to get them more easily, or at a less inflated price. (At my local Best Buy, the 1080s were the only ones consistently in stock on the shelves.)
1060s, 1070s, and 1080tis are pretty good. The important thing to consider here is rig density.
With some fixed number of slots, you can make better use of them with 1080ti cards rather than 1060s. However, the 1080ti usually does not offer the same individual price/performance ratio that you can get with 1060s. For example, I can get 3GB 1060s for around $200-$210 after hunting around. A 1080ti would cost me at least $700, and it does not mine at triple the rates of 3GB 1060s on any algorithm I'm interested in. (For example, I can get about 300 H/s mining Zcash with each 1060. I haven't seen many reported hashrates for a 1080ti that exceed 750-800, at best.)
With 1060s, you're getting a smaller overall rig hashrate for the motherboard/CPU/ram/etc. that you got to support the GPUs, so that's what can start to mess with the overall price/performance ratio when compared to 1080ti cards.
Another thing to consider is resale value. A 1080ti will most likely retain its usefulness for longer than 1060s, assuming you maintain them just as well.
1060s, 1070s, and 1080tis are pretty good. The important thing to consider here is rig density.
With some fixed number of slots, you can make better use of them with 1080ti cards rather than 1060s. However, the 1080ti usually does not offer the same individual price/performance ratio that you can get with 1060s. For example, I can get 3GB 1060s for around $200-$210 after hunting around. A 1080ti would cost me at least $700, and it does not mine at triple the rates of 3GB 1060s on any algorithm I'm interested in. (For example, I can get about 300 H/s mining Zcash with each 1060. I haven't seen many reported hashrates for a 1080ti that exceed 750-800, at best.)
With 1060s, you're getting a smaller overall rig hashrate for the motherboard/CPU/ram/etc. that you got to support the GPUs, so that's what can start to mess with the overall price/performance ratio when compared to 1080ti cards.
Another thing to consider is resale value. A 1080ti will most likely retain its usefulness for longer than 1060s, assuming you maintain them just as well.
You are right on card performances, but for the rig building cost, you should make it like: 1 rig of 6 cards 1080Ti need only 2 PSU 850W + 1 CPU +1 MB + 1 RAM..... while 3 rig of 6 cards 1060 need 3 PSU 850W + 3 CPU, 3 MB + 3 RAM... The peripherals' price is not cheap
For me, 1080 Ti is a good deal but I'm just started with a small budget, so I'm going with 1070.
I'd say you're definitely skimping on the PSUs there, especially for 24/7 operation, and even after messing with clock & power settings.
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