I haven't heard a comparison IRL, only over the internet, but they really do sound the same. Front panel is slightly upgraded on Alia with the new Envelope output (plus, I'm not a huge fan of the original cluttered panel artwork). Alia implements the same synthesis engine as Alter, just on a different chip. So it will react to your various inputs in pretty much the same way. It has the added bonus of being re-configurable to any other Alia module.

I haven't seen or heard any evidence that any part of the Alia is a downgrade from the Alter. Looks like every change is either lateral or unambiguously an upgrade.

Maybe someday BIAlter will have some kind of "collector's item/original version" cool, but it was already so widespread before Alia came out, I don't think it's rare enough to be that unique.