porcelain
noun [U]
1 a hard but delicate shiny white substance made by heating a special type of clay to a high temperature, used to make cups, plates, decorations, etc:
a porcelain dish
The tea cups are (made of) porcelain.
2 cups, plates and decorations, etc. made of porcelain:
He had a fine collection of Meissen porcelain.
china
noun [U]
clay of a high quality that is shaped and then heated to make it permanently hard, or objects made from this, such as cups and plates:
china plates
从以上这个解释来看，
porcelain侧重于工艺品类陶瓷作品,也是较为常用的"瓷器"的总称
china则侧重于非工业类(比如说日用瓷)