Series 8 and series 9 share lots of features together. The notable advantages of S9 are the following:
- double tap feature (series 8 supports something like double tap, but reports are that it's not as accurate)
- faster processor
- better screen (brighter in general and can dim more in dark places)
- on device siri
- precision finding (if you have iphone 15)
You should be the judge to see if these features are worth it. I think if you already have S8, it's not worth it to upgrade to S9. However, if I'm getting a new watch today, I'll most likely just get the S9. I tend to keep my electronic items a bit longer, so usually get the latest and greatest when I upgrade.