I asked Februs that question a couple months ago. Here is his reply:
> hello Februs:
>
> What is the difference between "Save and Reply" and "Archive and Reply"? I
> don't get it!
Februs Scripsit:
At some point we will do a cull of older messages and at that time
messages older than a certain number of days will get deleted.
However, if they've been archived then they'll be retained. It's
basically a permanent store versus a temporary store. | 
