If you are looking strictly at Ethereum mining then go with the 1070's.  All other mining I would go with the 1080's as you get better hashrate for about a better cost ratio and the resale will likely be much higher.

I'm currently running 7-1080's and 1 1080Ti and make on average about .023 BTC per day doing auto-exchange at Zpool either with their built in multialgo or using Multipoolminer.  You might be able to do better on specific coins, but only if you are willing to deal with trading them on the exchange yourself.  Life for example when Digibyte spiked I was pulling in about $120/day for all my rigs, but that was only if I dumped the coins at the high point.   You have to decide if you're taking your profits today or holding the coins you mine until the time is right for them.

Here is example hashrates for a 1080:  This is a single 1080 Zotac AMP edition

https://i.imgur.com/hwuXWIT.png

This is my 5 - 1080 rig - 2 Zotac Amp Extreme Edition, 2 ROG Strix, and an EVGA 1080

https://i.imgur.com/ghunKhp.png

There is quite a bit of variability in what hash rates you'll get out of each cards including power draw.  I notice those Zotac Amp Extreme Edition pull about 15% more hash, but also consume like 50W more in power.  The 1080Ti gives roughly about 40% more hashrate for about 25% more power consumption and 30-50% more cost. You have to decide if that is worth it to you.

Pure ethereum mining I'm seeing roughly like 20-25Mh per 1080 usually with no overclocking and setting a Power limit of 80%.  Setting power limit to 100% gives like 3% more hash, but consumes like 20% more power.