I have the S9 and previously I had the S8. I only upgraded because my daughter wanted my S8. The only noteworthy improvement on the S9 is that the finger print reader is much more reliable. A lot of people, including me, had trouble with the S8 finger print reader. There might be other improvements in the specs but in the real world they are not noticeable. Keep your S8 until you are ready for the S10 or S11.
I didn't even know it was wider until I read it on this thread. The difference is, as another responder so eloquently described it, just a speck. I can't see the difference, if indeed there is a difference.
I use both an S8 Plus and S9 Plus and honestly can't tell the difference now that the S8 Plus has been updated to Pie and Samsung One UI. If I really put them side by side and open apps the S9 Plus is slightly faster but defiantly not worth the money to change.