I haven't yet spotted a pattern for when to use which version! Does anyone have any suggestions?
According to the notes, they are interchangeable:
"A common alternative to cia mheud is cò mheud which means the same thing and occurs later on in the course."
Ah - thank you. I'm at the 'later in the course' and had forgotten that had been in already. Cò is certainly used more often in Duolingo. I wonder if that's the same in real life?
Cia is more common in day to day usage I'd say.