I just made a list below summarizing my experience with the two. I've using 1Password for 3-4 years now and still enjoy using it. Based on my experience (and sadly I'm not alone) iCloud Keychain works, but when it messes up and doesn't sync correctly, you may have to delete all you passwords to get it going again as I did.
Advantages of 1Password:
1. More Functional UI to manage/update passwords
2. Works across all browsers and platforms (For macOS - iCloud Keychain just works on just safari and doesn't integrate with any other browsers)
3. Less chance of loosing passwords/having issues during password syncing - This was the main reason I switched as I lost all my passwords on iCloud Keychain (Multiple times, and apple support is not helpful). 1Password really just works
4. Can be used to store secure notes, credit cards, database passwords, etc
5. Has web interface to view and manage passwords (if you use the subscription)
6. Can have multiple vaults to share or group passwords as you'd like - iCloud Keychain has no way to share passwords
7. Can generate passwords with any combination you'd like (For those sites that have special requirements)
8. Has 2FA support built in so you don't need another app to store them
Advantages of iCloud Keychain:
1. It's free
2. Works slightly better with safari on MacOS (With the new safari extensions coming might not be too different)
3. 1Password builds in a master password that you have to remember - iCloud Keychain is typically linked with your Apple ID
4. iCloud Keychain works better with Apple TV
Both
1. Both have TouchID/FaceID integration
2. Work seamless on iOS due to the passwords API
Wirecutter has a great article on comparing password managers as well
https://www.nytimes.com/wirecutter/reviews/best-password-managers