If you Export as Catalog, it doesn't matter if the photos are DNG or RAW or TIF or JPG or PSD. All of your edits and metadata will be in the exported catalog file. Should you ever need to restore from this backup, you use the exported catalog file (you can double-click on it; or you could use File->Import from Another Catalog). You would NEVER re-import the photos, your edit history and virtual copies and a bunch of other things (and possibly all metadata) will be gone if you do that.
By the way, its not a simple question, and one that a lot of people ask about.