The key differences between iCloud and Dropbox is that iCloud will sync the native macOS documents folder along with your desktop. Dropbox on the Mac has slowly been expanding its feature set and how it works, and today, the latest beta adds the option to sync your documents, downloads, and desktop.
The move comes just weeks after Dropbox reworked its core sync engine to make it easier to maintain and improve reliability.
Like with iCloud, this only really makes sense if you subscribe to one of Dropbox’s paid tiers (2GB is woefully inadequate). Even so, it could be very helpful if you want cloud storage that serves more as a seamless backup for common files than a place to upload specific files.