I prefer
Susan. In spite of the fact that it's dated as is
Helen, I don't see it as frumpy, while I do find
Helen very frumpy. Perhaps the latter impression is partially caused by the fact that my college sophomore roommate was named
Helen and she was extremely frumpy.
Even though
Susan has fallen farther than
Helen has, and even though
Helen's popularity occurred longer ago than
Susan's,
Helen still feels more dated to me. I can see
Susan on an eighteenth century woman, maybe that's why.
As for variants, I love
Susannah, whereas I only like
Susan.