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