First look on iPhone 8 and iPhone 7 comparison shows that phones have more in common than differences.
iPhone 8 and iPhone 7 have same:
size and bumping camera
photos (camera) quality
accessories in the box
both need cases
support wireless charge
have glass back
got screen auto white balance (True Tone)
As usual a little bit faster than previous model. Little bit heavier. Same size. Compatible with existing accessories. First quick charge test shows same charging time for both iPhone 7 and iPhone 8. Same need to use case because of bumping camera. Plus, another reason to use case with iPhone 8 is because of its glass back. It may be broken and it get dirty looks fast. Minus one reason for the case is less slip grip of iPhone 8. The phone is no longer try to fell from a hand all the time. When using a phone in case it is hard to say if it is iPhone 7 or iPhone 8. Screen scratch protection is still not perfect. Couple of days in pocket with coins and/or second phone – visible scratches. Same USB 2.0 wired sync speed. Type-C to Lightning cable is also USB 2.0 sync speed. Phone does not have Apple wireless charger yet, but Samsung and other Qi charges work for iPhone 8.
Now it is black again. Gold Black is best. Same Size as iPhone 6/6S. With improved antena section color design. Better camera that is shifted a bit to the phone edge.
New Home button. That is not actually button any more. Finger clicks work fine in general. Click in gloves or using nails do not work anymore. Fingerprint recognition speed and quality same at it was on iPhone 6S.
iPhone 6S looks absolutely like iPhone 6. Difference is in letter S an the phone back.
And color of camera lens. It seems like kind of optical filter added to 6S.
Touch ID 2. Sensor looks outside exactly like previous model, but work about twice faster.
Photo have better resolution (12MP). Video have better resolution (4K). SLO-MO have the same 240 fps at 720p.
New feature Live photo is quite interesting. In addition to photo iPhone store couple of seconds of video. Video is up to 15 fps, with 1440×1080 resolution. Can be imported on PC as a .mov file with same name as photo. Video MPEG4 AVC/1440×1080@Variable(upto15fps) Audio PCM/44/16/mono.
3D Touch is also nice. It is actually an another dimension of the user interface. Interface of 6S now react on how hard you push. It may be simple context menu or preview/open depend on force of a touch.
White balance of complex scene is much better due to two colored LED flash. So there is no more unnaturally yellow backgrounds on indoor shots with flash. See example below. Right is iPhone 5S, left is iPhone 5. Also note blue shadow behind the object. Two color LED flash has disadvantages.
An a everyday shots there is not much difference found. See example below. Upper image is iPhone 5S shot. Lower image is iPhone 5 shot.
Low light shots show some noticeable difference. See below. Upper image is iPhone 5S shot. Lower image is iPhone 5 shot.