Shape: Freshwater pearls come in greater variety of shapes (round, oval, etc) and colors. Satwater pearls are usually more lustrous and have a rounder shape.

Colour: Saltwater pearls usually have Gold, Black and White color while freshwater pearls have pink, purple and white color.

Freshwater pearls are cheaper for several reasons.

One of the reason, freshwater mussels are larger and can thus produce more pearls at a time as compared to saltwater oysters.
As a result, saltwater pearls are more rare.
Saltwater pearls are mostly round and have better lustre characteristics that are more sought after and therefore command higher prices.