1. The Snapshot basically takes an image shot of the data you want to backup as 1, whereas the Full backup is the actual process you use to backup that data.
2. I always use Full backups to restore data. Less tapes, means fewer tape changes, and quicker data recovery.
3. Symantec don't recommend doing AOFO with DB backups. This includes Exchange and SQL. I had seen AOFO causing errors on my Exchange servers where the logs weren't clearing, but this seemed to have been resolved with BE 11D rev. 7170, and I haven't had this problem at all on 34 sites using BE 12.5. With SQL however, I'd suggest not using it. I had to log a call with Symantec over an issue where I couldn't restore a production Navision DB to a dev environment, and they said with AOFO and the snapshot feature they used, it wasn't possible. I could only restore onto the same server, albeit a different DB. On this particular site, I now make do with Failed backups due to open files. You need to decide what's more important to you, and go that way. I haven't had an issue with logs not clearing with Exchange since moving to 12.5, using AOFO, and that's why I continue to use it.
4. I'd separate your DB and file backups out. Symantec will also inform you of that if you have any issues with backups/restores and they ask to see your selection list.
Hope it helps.
