I have a 6s and my missus has the 7. The differences are so miniscule it's hard to believe. Maybe some techy will tell me of all these amazing differences I'm not noticing. But I was taking pictures, using apps, listening to music on the speakers etc. and couldn't notice a thing.
Design wise it's pretty similar, same size just a little lighter (think about 5g). Few slight design changes like the antenna strips and headphone jack, as well as the home button being different. It also has waterproofing which the 6s doesn't.
Screen is apparently brighter so should look a bit better, but same dpi as the 6s. Both front and back camera have been improved.
Processor and GPU have been upgraded, the benchmarks seems to show a pretty big improvement but for general day to day use it's probably not something you'd automatically notice as not like the 6S is slow.
It's definitely not a massive upgrade though. I've got the 6 and skipped what should have been an upgrade this year as it didn't really offer enough of an upgrade for me to bother with and will wait for the 8 next year which is highly likely to be the big upgrade which is due.