Hi! Highlight changes is more of a visual indicator on the sheet (with a yellow highlight, for relative timing to show any visual changes). The activity log, on the other hand, shows a bit more depth and detail, such as viewership, changes made, automations run, and more.

For myself, I use the activity log to look at any long-term changes (and events), and occasionally export it for all details for the lifetime of the sheet.

Highlight changes, however, is super useful, especially in tracking changes within linked cells, or catching up on the sheet after last use.