I’m not exactly unbiased, as I greatly prefer the “export” workflow. But I’ve come around somewhat to Save As (actually, Save a Copy, because I really want to keep working on the .ai file).
The short answer why is simple: Save-as-SVG and Export-as-SVG should do the same thing, but they don’t. It’s like there are two independent pieces of code in Illustrator, with nothing shared between them. The Export-as code is newer and has traditionally had more bugs. Even in the current version, there is at least one situation where it produces the wrong output. In addition to that, the Save-as version includes a special header that says it was produced by Illustrator. The Glowforge software recognizes this and changes its behavior to improve compatibility with these files. The generic SVG produced by Export-as won’t trigger that. Finally, since Glowforge does all their testing on Save-as generated files and only supports this workflow, you’re probably going to have an easier time of it.
I guess that answer wasn’t as short as I hoped.