My understanding is that Agave vivipara is a separate species with a different growth form, but the name has been misapplied to Agave angustifolia, and is in that sense a synonym when referring to Agave angustifolia.
The blue-green form of Agave angustifolia (known as Maguey Espadin) is apparently the most commonly used Agave to make Mezcal. [Tequila, a specific type of mezcal, is only made from Agave tequiliana weber blue .] Search for Maguey Espadin and you'll find numerous pictures.
Generally the name Agave angustifolia is generally used as if synonymous with the variegated form, but there are numerous references that cite 'Marginata' as a cultivated variety of the species. It even appears that 'variegata' is similar to but visually distinct from 'Marginata'. From what I have read, 'Marginata' is by far the most common form in landscape use.
Some links:
http://books.google.com/books?...
http://keyserver.lucidcentral....
http://en.wikipedia.org/wiki/A...
http://www.elginnursery.com/pr...
http://www.public.asu.edu/~cam...
Still, I'm no expert. I'm only offering a suggestion based on my understanding from my research before adding it to my personal plant database some months ago. I wanted to post here so others could freely add their input.