It's not that I think her acting is better-- it was great as the savior and it still is-- but I just feel like acting evil is slightly more difficult than acting good.

I think both!

Actingwise, Dark Swan by far.

Neither. Jennifer Morrison's best performance was Zoey Pierson in "HOW I MET YOUR MOTHER". Her portrayal of Emma is okay. But I find her portrayal of "Dark Swan" (good grief, what a moniker!) a bit flat and dull.

It's always harder to play the villain.

She looks really stiff either way, but more so as the dark one

I like her more as the saviour, but I think playing Dark Swan requires more acting skills.

Even when she's the dark one, she is still the savior

It's more natural.

I hate her Dark Swan performance. It's so stiff and dull, and her voice sounds really weird.