I upgraded from the S8 to the S9 only because my daughter needed a new phone so she got my S8. The differences are few but I think they made some internal upgrades that just makes everything work smoother. The most obvious change is the finger print sensor. The new position makes more sense although after over a year of using the S8 it sure took a while to get used to the slight change in position. Many people, including me, with the S8 had trouble with the sensor working. I used to delete my finger prints every few months and add new ones. That hasn't been a problem with the S9, at least not yet. It also seems to recognize my finger print faster than my S8 did.
Supposedly the camera is better but I haven't really noticed much difference.
Battery life was pretty good on the S8 and seems to be better on the S9. I don't need or use the always on display so that probably contributes to the better battery life.
The best way to improve this phone would be to eliminate the Bixby button (and Bixby too). If they won't remove it Samsung should at least make it so that we could reassign what that button does. It would be pretty cool to have an extra button that could be re-mapped to become a shortcut to do almost anything. That would make the phone stand out.
The S9 is basically an S8 with some minor tweaks to improve known problems. I expect that I'll be happily using this phone for many years to come.