Costa Rica is a wonderful place, too! You should definitely go if you haven't been. The people (Ticos) are friendly and warm. The weather is great and it's a beautiful country! Definitely go!
I love both Countries , but if you have already been to Guate and never to Costa Rica i would go to CR
Have you thought about Honduras? There are places here in need of someone like yourself.. and it is very beautiful.. Caribbean beaches and rainforest covered mountains. Anyway, check out my link to some organizations here that you may be interested in helping :)
I've been to Guatemala once before and I'm going again this December. (Mission Trips) but I have a chance to go to Costa Rica in march or go to Guatemala again in June but with a different organization. I love Guatemala soooo much and the people there but maybe I should go to Costa Rica.