The first and most important aspect of choosing is to ask yourself what your needs are. How often do you have guests, how many guests do you have, and will they stay overnight? How often will this sofa be used as a seat? Is it for your living room or for another room in your house? Will it fit in that room? For example, if you have only the occasional guest, you may purchase a longer sofa that can be slept on.
Any sofa you are considering needs to be sat on before purchasing to ensure it is comfortable. This is especially true of a pull-out sofa, which can sometimes be comfortable to lay on, but not comfortable to sit on, or vice versa. Both sofas and pull-out sofas should also be laid on to ensure they are comfortable in that respect as well. If you can feel the frame when sitting or lying down, you should consider another sofa.
Check the construction of each couch before purchasing. A pull-out sofa should be made of hardwood or steel; it will need to support someone's weight for far longer than your average sofa. It should also be easy and quick to pull out. A standard sofa can be lighter, but should still be made of hardwood. Ask about the springs of the sofa, and see how it is assembled: It should be held together by screws and brackets, dowels or other solid construction.
The final consideration is how the couch will fit into your home. For example, with a pull-out sofa, you will need to leave enough room in front of the sofa to make the bed. Any coffee tables, chairs or other furniture in the room will need to be light enough to easily move out of the way. Also, check the available upholstery, and try to see the couch fully upholstered in your chosen style to ensure it is exactly what you want in terms of decoration.