From the 8 line to the 9 I wouldn't say much, but from the 7... Phew! You have a ton!! Better battery life, HDR/Dolby support, newer software, much faster processor, more RAM, and of course, the much improved camera compared to the S7 (with an additional 2x lens if you opt for the plus model). You also get the new Samsung Experience but if you use a different launcher already that might not be a problem.
You will get a better sound experience when user the phone's speakers.
Can't remember if the S7 was still 16:9 or not, but that's also a factor to consider: the screen ratio is now narrower, and that puts some people off.