They can both give soft results and both have advantages/disadvantages.
Umbrella - works well with both hot lights and flash, cheap, compact, less controllable than a soft box (more spill). Can give odd shaped catchlights in eyes and highlights on shiny products.
Soft Box - need specially designed ones for hot lights (heat doesn't dissipate as well), expensive, more time to set up and break down, bulkier and heavier than umbrellas during transport, more controllable, accepts grids for even more control, gives rectangular catchlights in eyes and highlights on shiny products.
