I've never watched it, but I wouldn't describe it as a soap. for one thing soaps run continuously, and not in series'.
I'd call it a costume drama.
They are more or less the same. Both are described elsewhere as period drama's and bothe are set in the run up to the first World War through to the interwar years.
I think the essential difference is that upstairs downstairs is set in the city and the other one isn't.
I've not watched it either but agree with Trevaunance that its a costume drama...
I have to admit ... I don't watch much TV but Downton is one of my guilty pleasures. I'd agree, costume drama.
Historically anachronisticc aristocratic arse licking shite ...... BB loves it