Have stayed in Cadiz - which has an old town and new town. I prefer the old town and have stayed in the Parador there which overlooks ocean - its simple and a typical parador. Very quite at night, bar not open after midnight. Most of the hotels are in the newer part of town which is quite sprawling along a beautiful bay and they all look nice and would probably have ocean views. Good shopping in new part of town with a large shopping centre.
Jerez is very nice too - have done sherry tour there, all the major sherry houses have tours and its an interesting thing to do. Its smaller than Cadiz and equally nice in a different way.
You could stay in either Cadiz or Jerez and visit the other one if you have car, they are close. Have driven through El Puerto de Santa Maria and its much smaller - would think you are better staying in either Cadiz or Jerez. The coastline is lovely along here with fabulous beaches but it is very open and can be very windy even in high Summer.