I spent most of my time learning Spanish in both Antigua and Quetzaltenango (Xela), but you can learn pretty much anywhere in the country.
I talked with travel bloggers and spoke to people I met on my travels who have done it, and this helped me make an educated decision.
At both my schools, the Spanish teachers were hugely experienced. Both had more than 10 years of teaching Spanish behind them and it showed.
This means choosing a place to stay for an extended period of time. You will want to decide on this carefully as you don’t want to get stuck with a nasty hotel room with bugs for a week.
Price may or may not be an issue for you, but I would suggest going with the best Spanish school rather than going with the cheapest.