Is it worth staying at a guesthouse instead of a hotel at a tourist destination?

It depends more on what you're looking for than on the price itself. A guesthouse is better if you want something more intimate, with personal service and local tips from the owner, but a hotel usually offers more amenities and the security that everything will work properly. If the destination is small or you want to experience a more authentic vibe, a guesthouse is definitely worth it; if you want convenience and services like a pool or room service, a hotel comes out ahead.