I'll be taking a course for two weeks beginning next in Mexico.  I'll let you know how it goes.

I've never been in Guatemala and I am from Spain, so my opinion will probably be biased. As Michael K said, I think the most important thing is to look for a good language school and check people's reviews/opinions. Prices, accomodation and other activities apart from classes at the language school, are also important. 

I did a course in the UK many years ago, and the most important thing were the activities the school organized out of it. I mean excursions. After all you can attend Spanish classes in your home town or even talk to Spanish speakers there or online. I think there are so many different things to take into account that it is not just going to a country or another.

I recomend you Spain, basically because I live in it and it's beautiful (you'll love our food). You also have to keep in mind the differences in the lenguage between Guatemala and Spain. Anyway you'll learn and have fun in both of them for sure. 

They have the same languague but there are many vocabulary and expressions that are different.

I have attended Spanish language schools in Spain and Colombia. It can be a very positive experience. I do suggest that you look at online reviews and check the schools facebook page before choosing. TripAdvisor is another site that can help with that. A good school should be well liked and have an active and open facebook page.

I went to inHispania in Madrid for a week and liked it. I am sure that there are tons of other immersion schools in Spain though.