Stay with iPhone. Samsung is going to be premium with the Note 8 anyways, so you might as well get the best bang for your buck. iPhone will have much more value for you because of iMessage and being able to use your same apps from the 7 plus, meaning the ecosystem continues to provide benefits.
With the S8/S8+, you have trouble getting screen protectors. You will also see aggressive price drops in a few months, so buying the Note 8 early will hurt a bit. The Note 8 could feature similar Samsung lag because its software is notorious for it. Their customer service is subpar. I don't expect the battery life to be good either.
Apple will close the hardware gap with the 8 since it will have an OLED screen, Face ID, small bezels, quick charging, wireless charging, a higher screen resolution, better water resistance. That's in addition to dual stereo speakers, the best processor on the market's successor (A11), clean software, tremendous longevity with app updates and software updates, top tier customer service, plenty of accessories, and iOS 11 narrowing the gap between itself and Android.