"Clone" for both DVD and Blu-Ray makes a 100% identical image of the original disc after it is patched to remove protections. This keeps all file system metadata intact, including all original dates and file times, disc IDs, authoring application IDs, and full folder structure. It is the best way to make a backup be as accurate as possible to the original, if that is your goal. You will need a dual layer blank if the original is dual layers.
"Full disc" is a reauthoring option. Even if you are not compressing the video to make a dual layer fit on a single layer, and even if you are keeping the original menus, this is still a reauthoring option. It creates entirely new file system info and metadata, and scans through all of the original video. Most people use this I believe if they want to try removing content (such as bonus features, trailers, etc.) to try to get just the movie to fit on a single layer blank without needing recompressed. Some people like their backup to not show any warnings or advertisements.
You are right, if you do "full disc" and remove nothing and compress nothing, then the disc will behave the same as "clone," minus the original file system info and metadata, and possibly some folder content that isn't part of Blu-Ray or DVD standard structure. You will still likely need a dual layer blank if the original is dual layers. In short, "full disc" without removing anything and without compressing anything makes no sense to do in place of "clone."
In my opinion, if you are using dual layer blanks when necessary and if accuracy to the original is important, "clone" is the only option. ;)
