I just got Spider-Man and nova the other day. They both play great. I'm selling Nova, because I don't care for the character, but he is really fast and feels powerful. And oh Spider-Man is a perfect fit for this game. It is insanely fun to swing around a toy box with him. The nailed his character and powers.
Black Widow just doesn't have the most exciting abilities in general, as a character. I love playing as Hawkeye though. His bow attacks are incredibly powerful.