I picked up the S8 and also have the iPhone 7, after setting up the s8 ive been comparing battery life to the iPhone 7 and trying to use both equally and so far the iPhone has better standby. The s8 display looks bigger for sure but when you compare the phones side by side the s8 is only about 3/8" taller and almost identical width compared to the iphone.
I listen to podcasts in my office and it would be hard to tell which phone is actually louder but both sound good. Its nice to have a headphone jack on the s8 and both phones sounds equally good with earbuds. Ive taken pictures with both and during normal lighting conditions they are pretty close to each other but the s8 has colors that stand out a bit more. The s8 pulls slightly ahead in low lighting, the iPhone takes better steady video while walking so they trade punches in different scenarios.
Reception in my location on at&t is pretty much even, however my Lenovo laptop instantly tethers to the s8 where the iPhone sometimes requires me to restart tethering in order to connect properly.
I wont go deep into app differences but the weather channel app is butter smooth on the s8 but slightly hesitant on the iPhone, and google maps seem to be slightly smoother on the s8.
overall I am not big into customizing my phone anymore I pretty much set them and leave them, using them back to back I would have a hard time deciding which one to keep.. The biggest standout IMO is the s8 display, all other aspects they are pretty close.