Practically, though, the difference is minimal. The "ambassadors" are
just the tech-savvy non-English-only interested users who spread the
word in the non-English Wikimedia world, which is exactly what happens
with any announcement English-only list.
The two differences I see are that wikitech-announce is meant for
developers rather than users and that -ambassadors is a "lowest traffic"
list. I don't know if it's worth it, in any case please please describe
their scopes very well: many mailing lists are not necessarily bad, but
they surely are if their scope is confused.
Nemo