I've had iphones since the 3gs after a short spell with a HTC. I've just sold my 7+ to my brother with the intention of buying an X but having just been in the apple store and tried the 8 and the 7 side by side the 8 is actually slower at everything I tried. I.e., app switching, loading new apps, typing etc. Shocked is an understatement. I took my brothers old 6 off him as a trade in and in the process of wiping and restoring the phone I was forced to upgrade to ios11 on it. It is now a complete pile of cr@p, it's woefully slow and reminds me of an old android with all of the stuttering and lag. My point being that I don't think the 8 was at fault rather the fact that it runs ios11 out of the box.
My 7 had been perfect for the last year, fast/smooth/reliable and i'm now regretting selling it. With all of the new tech in the X and the fact that it runs this ios11 junk, i'm now not buying it at all. I even have the day booked off this Friday to go pick it up at launch, definitely not going now.
If I was in your shoes, I'd try to get a 7/7+ and hope that it's still on ios10 in the box but DO NOT upgrade.
I'm currently pricing Samsung S8's on ebay etc as I'm absolutely disgusted over the lack of effort on Apples part with this new software and the rip off pricing getting completely out of hand.
Good luck with your choice.