I believe it's better to go S8 Plus. Outside of the processor, I think the S8 Plus wins in most regards. If you're in the States or a Snapdragon region, that processor will not be as great as an Exynos. The AMOLED screen is ridiculously fantastic. 4gb RAM may not sound as high as the OP3 or 3T, but it is sufficient enough for a flagship device.
Enhanced biometrics (as soon as Samsung patches the facial detection) is huge plus for me. I rather have an always on fingerprint scanner and Iris Scanner combo than just Touch ID 2.0. Samsung Pay blows Apple Pay away.
The S8+ camera is better than the 7+'s dual camera module. The front facing camera is better as well. Snapchat is getting better on Android as well, so you can take the chance on the S8+.
Android is better because you can do more than just confining yourself towards what Apple wants. I own both a high end Android device and iPhone SE.
The only perks I get from iOS are the following: more reliability at core functions (texts, app updates, and software updates), iMessaging/FaceTime, better battery life with stronger RAM management, and stronger customer service and device support.
Meanwhile, my S7 provides me with better hardware, more features, and a lot better customization options. My S7 would be even better if the battery life was great with standby battery life and overall battery life. With the S8+, you don't have to worry about battery life.
With the S8+, you have a great known. The iPhone 2017 is an unknown right now that could go in various directions. I don't have faith in Tim Cook to sign off on a Galaxy S7 Edge or Galaxy S8 caliber iPhone.