The first thing to consider is how comfortable the furniture will be as a bed and as a couch, so you should always test the comfort level of the furniture in both forms. Many people consider futon mattresses to be very comfortable, but others think they are too hard to sleep on because they have a thin mattress. Sofa beds are generally made of thick furniture foam, so they may be softer than a futon mattress. If you will be using your sofa bed more as a couch than a bed, you should also pay attention to how comfortable it is for lounging rather than sleeping.
Futons are generally more affordable than convertible couches or sofa beds, but as with any type of furniture, you get what you pay for. If you are considering sleeping on a futon more than using it as a couch, then you want to get a higher quality (and higher priced) mattress, with foam inserts and other comfort features. Sofa beds can be more expensive than futons, because they are often made with high-end furniture foam and fashion fabrics.
Futon beds fit well with Japanese-inspired, eco-friendly and modern decor styles. Sofa beds, also called convertible couches, are a modern type of furniture, and as such, they can fit very well with a modern style home. For more traditional or Victorian-inspired decor, a daybed filled with pillows or a traditional sofa bed that has a mattress that folds back into the cushions might provide a better look.
Another consideration when choosing between a sofa bed or a futon is how easy the furniture is to convert from a couch to a bed. Many futon frames require two people to lift and pull the frame into a flat position for sleeping. A convertible couch may be easier to fold down into a bed, because the arms and back of the couch simply have to be moved into a flat position. Unfolding the mattress from a traditional sofa bed may also be more complicated than either a futon or a modern convertible couch.