Both would have 16gb of ram
Depends on what you do with it.
The i7 is a little faster than the i5:
And the 1080 is about 25% faster than the 1070:
Do you need a faster GPU more or a faster CPU?
The GPU is easier to upgrade because they are all compatible with the PCI Express x16 slot and you don't have to mess with thermal paste, so I would probably get the faster CPU then upgrade the GPU when it's needed.
Both 1070 and 1080 are enough for any games or applications at the moment. The i5 is fine for any gaming, but for tasks such as video editing etc, get the i7 to be more secured.
What are you going to do with it? If you're just gaming, get the i5 and 1080. If you're gaming and livestreaming/making videos, get the i7 and 1070.
I'd do the i7. Either video card can play anything out there right now at max quality, but using an i7 means that CPU intensive apps (like anything procedurally generated) will run faster. Also, if you multitask (i.e. web browsers, skype, ventrilo, and an MMORPG all at once with a NES emulator open) that's i7 territory.