How does iCloud Drive compare?
How does it compare to Dropbox as a sync service?
And can I keep a copy of my files on my Mac at all times if I'm using iCloud Drive?
Question marked as
If you don't turn on "Optimize Mac Storage" in Apple menu > System Settings (System Preferences in macOS before Ventura) then there will always be a local copy of your iCloud Drive files.
For Photos, if you don't turn on (in the Photos app) Photos > Settings (or Preferences) > iCloud > Optimize Mac Storage, there will always be a local copy of your Photos.
I haven't used Dropbox recently enough that I can compare.