Huge difference to me.
I went from 6 Plus to 6S Plus to 7 Plus to 8 Plus. And I noticed a huge change going to the 8 Plus from the 7 Plus.
The speed is noticeable. I mean the geek bench numbers stand for themselves. It's just completely smooth and even smoother than the 7 Plus, which wasn't/isn't a slow phone by any stretch. But the 8 Plus blows any current smartphone I've tried (all the iPhones, Note 8, Pixel 2/2XL, S8, LG V30, etc) out of the water by comparison. And speed is my number one thing I look for in a smartphone. No lag tolerance here so it's gotta be right as rain.
The camera is something I appreciate as a hardcore Instagramer/Snapchatter. Portrait Lighting is very cool, but not everyone can appreciate that. For me with just general pictures there is a small but appreciable difference.
Wireless charging, OMG, that is almost worth it alone for me. See, I normally at bedtime shut the light off, play with the phone for 30 minutes, and then put it on the Lightning Dock, fumble to get the stupid port to line up with the connector in the dark for 2 minutes, and then go to bed. Now my routine is shut the light off, play some games for 30 minutes, and set the stupid thing on the charging stand and call it a night. No more fumbling. And I want my phone upright and not just a cable so I can see my notifications first thing in the morning and the time. A cable would be easier to fumble with in the dark though than a dock, but I want it upright so I dealt with it until wireless charging.
So I don't care if it charges slow over wireless, I let it do it overnight while I'm counting sheep and it's magically to 100 percent every morning. I'm looking forward to the OTA update though for faster wireless but since my battery lasts all day there's no need to top it up mid day. Fast charging with a cable is nice though, it is a welcome addition but I find I never use it because I charge once a day and that's always overnight with the wireless charging stand.
Definitely go 8 Plus.