renting a car in guatemala

You can get around with ease without bothering to rent a car. There are taxies, shuttle vans, big buses and of course the not recommended chicken bus. You can ride with a group of other or have a private car and driver. I find it best to have a local driver as they know the drill.. I never drive myself in Guatemala as find it nerve wracking. You need to be hyper alert as turn signals don't work and neither do tail lights so a lot of guess work involved. I also find some folks just have no idea how to drive at all. I would just let someone else worry about.

You can rent cars in Antigua and Guatemala City. I paid around $40 USD for a small sedan and $90 a day for a large 4x4 SUV last time down. Your regular driver license is enough to be legal to drive in Guatemala. Guatemala City is a hectic city to drive in but really no worse than any huge city. The countryside is pleasant driving, the roads are pretty good for the most part. Watch for washouts, washovers, rocks in the road and slow moving things like motor scooters and bicycles.

I use the common carriers for the most part to get around but to be honest they can be a pain. When I travel to Peten, I rent a car, there are just too many things to stop and look at to be on someone else's schedule. Lake Atitlan, I take a shuttle, Mixco Viejo, I rent a small car, the Cuchumatanes mountains, I rent a 4x4. A trip to the beach, a shuttle, the little towns about Antigua, a chicken bus, the museums in Guatemala City, I hire a driver and car.
A good map is important.

On a low budget with two people who aren't familiar with the country, I'd personally suggest against it and just go with shuttles. There really isn't much need for one unless you have special requirements.

We travel by public transport: chicken buses, pick-up trucks, minibuses, large buses. Added to that the odd shuttle and taxi. Shuttles can be more convenient as you don't have to change buses (but it is not hard and even easier for you as you speak Spanish) and although they can be cramped as well you will always have a seat.
But we enjoy the buses: we love the colours, the noise, the music, the hustle and bustle, the constant stream of vendors (and yes, the preachers who sometimes turn out to be vendors as well), the contact with the locals...
Also as you are on a budget... the buses are cheap. They are not always full but of course they often are and being packed together is an experience as well (sometimes kids end up on your lap). We like the agility of the ayudante and the way he remembers to give you your change even if it takes a long time. We enjoy the way they call out the names of the cities and towns. And they are always in such a rush (the drama of it all !) : we have climbed in the back while the bus was already moving. Ayudantes have run off with our backpacks to throw them on top while we try to hop in. Changing buses at Los Encuentros involves yelling, pointing, running. If you like people watching you will love chicken buses.

Most visitors don't rent cars, its a easy country to get around via tourism infrastructure and use shuttles, buses, boats and taxi hires.

We did rent a car in Antigua to go to the very end of the Pacific coast at Las Lisas, otherwise the other 21 days we toured the country with no rental. You don't need one or want one at Lake Atitlan, get around via boat. Roads are mainly two lane and many are a slow go. The chicken buses are nuts and I would avoid them too...

I don't think renting is a great idea if you are on a low budget. Public transport is cheap and plentiful....

The chicken buses are nuts and I would avoid them too...

No need to do this. The chicken buses (called camionetas) aren't "nuts," they're the backbone of the country's transport system. Easy to use and generally safe. Do they sometimes have accidents? Yes, but so do shuttles. For a budget traveler, they can be the best and most interesting way to get around the country.

the not recommended chicken bus. .

??? by who? Chicken buses are great and by far the cheapest way of travelling locally in CA.

Those buses flying at speeds over speed limit in the mountains and the suspension maxed out on turns is not for the faint of heart....they drive like its a F1 race...