The market in Antigua, i.e. the one where most of the locals go to buy certain things, is located around the chicken bus terminal. It basically consists of a few streets and alleys lined with stalls selling clothes, stuff and food etc.
The market is nothing spectacular and you just kind of walk into it, but I found it interesting, especially in comparison to the Mercado Artesanal, because you can clearly see the difference between a tourist market and a local market.

Go or not? I would just stroll through it if I was in the area anyway. After all, the Mercado Artesanal is very close by, and the bus terminal from which you can travel to Guate City for example for a day, is right there.