I wouldn't make that switch unless I wanted a much smaller phone. Which is what I did by getting the 4.7" 7.
I was able to keep my HTC 10 as a back up and switch out to it sometimes for a change. That'd be a nice option if you're able to do both.
Odd question. I've had most of the iPhones except for the 5C and SE. I'd love to try out the S8 but wouldn't touch the tiny older SE. They're not even in the same league. If you want to move to an iPhone go for a 7 or upcoming 8.
The only reason why I could see someone doing this is if they wanted a phone to be a bit more mobile, easier to put in the pocket throughout the day, use with one hand, maybe better battery life. That said, I use my iPhone 6s+ as a book, planner, note taker (with bluetooth keyboard), and communication device. As much as I would like to go back to the smaller phones (loved the feel of the 4s), the way I use my phone, I don't think I could.
The S8 and + are miles ahead imho in terms of display and battery life.
I decided to try the S8 + but returned to the iPhone because it was easier to facetime the babysitter at home with the kids.
I returned to the SE (spare device) as I sold off my S+, if it wasn't for the easy use to contact home while at work (Facetime is just simpler for the babysitter to use) I'd still be with the S8+