It truley depends on what you are looking for. If you are looking for a town that is true Spain, with true Spaniards living there, then Cadiz is what you want. If you want to be around the "well to do" and "wannabe to be famous" then Marbella is what you are looking for. I can remember when Marbella was a small fishing village, as late as the early '70s. Now I would say it is for the people that think they want to be in Monaco, but just haven't made that kind of money yet.
Go ten miles either side of Marbella and you are back where the real people spend recreation time.
Cadiz would be my choice between the two. Another nice town in that general area is Ronda.
Where ever you go you are sure to have a good time, just bear in mind that Marbella is full of phonies with a hand full of credit cards. :p