I last visited Cadiz in July, 2002 just as the Summer season was starting (16 Jul: Virgen de Carmen). It's still a very charming, bustling port city but I found prices there significantly higher than over in Huelva.
I expect it may be the same around Christmas because many people spend the Holidays there.
The old part of the city is still intact but there's been a lot of high-rise condominium-type (and resort hotel) construction along the coast just north of the city.
disclaimer: My evaluation is based on a one-day visit (on the ferry) from
el Puerto de Santa Maria where I was staying for a week.