There are several factors you need to consider before choosing which method you wish to use. By default, vBulletin stores attachments in the database since every server that supports vBulletin will work with this method. Not every server will be able to store attachments in the file system.
Storing in Database:
- Backing up your database, also backs up your attachments
- Does not consume any of your file system storage limits (unless mysql storage counts)
- Can increase server load on large forums
- Better performance for large forums
- Easier to backup your database
- Does not work if SAFE MODE is enabled on your server.
- Need to backup attachments separately from database
- Consumes space on your file system
Attachments as files seems to be the faster option of the two when you are dealing with a large number of attachemnts.
Staff comment: Actually that is not correct. The attachment itself is stored in the file system. Only the attachment info is stored in the database (size, post ID, user ID, etc.)