Just remember that car rentals from the country are usually cheaper than those at international companies. If you are traveling outside of the country, check with the company to see if you are  permitted to take the car outside the country; otherwise you won't be able to cross the frontier. Not all the companies allows their cars to cross the boarders, so call them before you rent your car to be sure. 

Just remember that driving in Guatemala is not like driving at home, so please be careful. Pedestrians cross the streets from any place, dogs stand in the middle of the highways (probably waiting for you to run over them!), children make the streets their football and soccer fields, traffic is heavy and there are people all over the place.  Here we list some of the car rental companies located in Guatemala City.