Pending on your needs.
Android 15 does run smoother because the display refreshing more ofter to higher settings but the battery drain is also higher. Also Android switch to Vulkan by default for better use of the phones hardware. There are some other under the hood changes like how app caching now works and defragmentation. So apps launch slightly faster.
For new features they are mostly basic things with AI like the call translation and circle to search. There are no new features ported from the ROG 9 series back to the 8 series yet.
Overall the phone feel faster but its also drains more.
