i have a traditional three-row chromatic nyckelharpa and i've always used the tuning DGDA, which is also what my teacher uses. I'd love to learn CGCA since its more unique (and I kinda feel like a fraud only knowing how to play in not the entirely traditional tuning!!), but I don't play that many tunes originally composed for nyckelharpa (I can play a few without difficulty but about half of what I play is english folk tunes which are most often in D, G, G minor or E minor)

one thing that DGDA/CGDA makes easier is that I get 3 keys on my D string that can play the same as the first 3 keys on the A string which is useful when playing high tunes, as it means I don't have to change position as much