Step 1: choose your shape
Step 2: choose your size
Step 3: choose your fabric
The thing to remember when buying a sofa bed is that it is a compromise. It is not a bed, nor is it fully a sofa. It is a hybrid and like most hybrids it is not quite as good as either of its starting points but it wins because of its versatility. If you have a tight space and need to offer both sitting and sleeping options, then the sofabed is the go to choice of most people.
Sofa beds will always feel a bit more firm than a sofa, because you are not sitting on springing but on cushions on top of the sofa bed action. They also cannot be as delicate in design terms because the shape has to disguise the sofa bed action inside. The best designs are those that have a valance as opposed to being up on feature legs.