1080 is a higher end card than 1070, so if the price is exactly the same I'd say go with 1080. The only argument in favor of 1070 ti is the standard GDDR5 ram which means higher hashrate in ETH and Neoscrypt. We can safely forget about ETH cause if you're mining ETH with 1070s then you're doing it wrong — it's ~ equal in speed to AMD Polaris cards that cost 2 times less; there's just always a more profitable coin for higher-end nvidia cards to mine. With 1060s yeah, ETH might be OK, but 1070s are wasted on ETH. This only leaves the Neoscrypt. That is the only algo that I know of where 1070 Ti outperforms 1080. Every other algo out there is faster on 1080, and there's plenty of them.

I'd take 1080 in your case. If it would be noticeably more expensive then sure, 1070 Ti is a better deal. For the same price, 1080 makes more sense. They're pretty close in terms of performance though.

I'll recommend 1070ti all the way!
I have both of them rocking in the same rig.
They perform close to each other in all the algorithms that I've tried but 1070ti also rocks in ETH where-as 1080 does not.

It depends on what algorithm you'll choose. For instance 1070ti way better at Ethash and generally consumes less power.

I'd say go with 1070ti
the difference is too small in performance but price wise the 1070ti looks much better
also note that in neoscrypt algo it will outperform 1080 and nowadays there are a lot new coins on neoscrypt.