For starters both are amazing dramas in their own way and I LOVED both these dramas!

Similarities:
~ Tons of plot twists
~ Keeps you on the edge of your seat
~ Fantastic acting
~ Fast-paced
~ Always something happening in each episode
~ Lots of secrets that get revealed

Differences:
~ W is more romance-driven whereas Circle has minimal romance
~ Circle is more sci-fi/dystopian whereas W deals more with the webtoon/cartoon realm and the idea of two separate dimensions