I used the carpet/upholstery cleaner from wilkinsons and it worked a treat on the little Peugeot I am preparing for sale. The seats were minging with suspected beer/coffee stains. clean as a whistle now using a soft nail brush also to rub the foam in, then wiped with a cloth. Job was done in an hour. I cannot see any benefit from a steam cleaner as I reckon you'll still have to put in some elbow grease if the seats are grubby.
Never use spray dash cleaners, they do not clean well and just seal the dirt in.
Do you mean a machine or a chemical?