The grade is about the fabric, not the sofa: the frame, the cushions and the size stay the same (sample guide).
Grade A cotton blends are hard-wearing and the covers wash at home, which suits a family room. Linen and wool look softer and cost more; velvet and leather cost the most and change with use, which many people like.
Ask for up to five samples by post and look at them in the room in daylight and at night before you choose.