Just to clarify, porcelain and stoneware are different types of clay. Either can be fired to a high enough temperature to vitrify, meaning they will be impervious to water. When something breaks because of a freeze, it is usually because the water in it has expanded when it became ice, and broken the rigid structure. Porous (unvitrified) ceramics can have water in them, and therefore can break because of freezing regardless of shape. Vitrified ceramics, neither stoneware nor porcelain, should not break simply because you freeze and thaw them. However, as mentioned, the soil (or water, or other water containing substance) contained in a ceramic container, can break even a vitrified ceramic if it doesn't have room to expand on freezing. Thus incurved pots are at high risk, while outward sloping pots are safer.
There are differences between porcelain and stoneware which might make them more or less prone to breaking, but generally, it's shape and vitrification.
Eg. Porcelain can be, and often is, thrown thinner, which would generally make it more fragile. Stoneware tends to be darker in color, which could cause it to absorb more sunlight and heat up quicker, leading to rid changes in temperature, which could cause damage even without water expansion, but these are relatively unlikely situations.
