they are very close in definition. See what the Webster dictionary has to say:
Definition of WELL-BEING : the state of being happy, healthy, or prosperous
Definition of WELLNESS: the quality or state of being in good health especially as an actively sought goal
Wellness, however, is a little narrower because it usually refers to health. When I say to somebody that I have a sense of well-being, I am describing mainly emotional factors which can include peacefulness, happiness and a sense of being one with the universe.
Thanks for asking and stretching my mind to reflect on your question.