Here's my take:
The iP7+ has better build materials, a better design in general (if you like a flat 16:9 display), a more accurate display, better battery life, better security out of the box and with updates, more storage options, will retain it's value better and will be updated and supported longer.
The S8+ has advantages with a larger, higher resolution AMOLED display, no longer has the FPS wasting space on the front of the device, a better camera, seems to be better ergonomically, has a slightly higher water resistance rating, wireless charging, higher base storage and with various deals, is substantially cheaper.
Really depends on what features are more important to each person, I'd be pretty split between the two, but the security and update support are pretty huge issues to me, so I might lean more towards the iP7+ than the S8+, but in June 2017, there's no way I'd spend money on an iP7+ today - we're only 3 months away from the beginning of our final round of devices.